diff --git a/.env.example b/.env.example
index 64ab490..d8921f0 100644
--- a/.env.example
+++ b/.env.example
@@ -172,6 +172,10 @@ VPN_CONFIG_KEY=
 # (WireGuard-Subnetz). Alles andere sieht die Platzhalterseite.
 TRUSTED_RANGES=10.66.0.0/24,127.0.0.1
 
+# Umsatzsteuersatz des Verkäufers in Prozent. Preise in der Konfiguration
+# sind NETTO; die Oberfläche weist beides aus.
+CLUPILOT_TAX_PERCENT=20
+
 ADMIN_HOSTS=admin.dev.clupilot.com,10.10.90.185,localhost,127.0.0.1
 
 # ── Nextcloud blueprint template ─────────────────────────────────────────
diff --git a/app/Livewire/Billing.php b/app/Livewire/Billing.php
index 98caa46..f9fc2a5 100644
--- a/app/Livewire/Billing.php
+++ b/app/Livewire/Billing.php
@@ -54,6 +54,21 @@ class Billing extends Component
             ?? $customer->orders()->latest('id')->value('datacenter')
             ?? 'fsn';
 
+        // A cart cannot hold two plan changes: they contradict each other and
+        // no checkout could resolve which one the customer meant. Choosing
+        // another simply replaces the pending one — add-ons still stack.
+        if ($type === 'upgrade') {
+            $replaced = Order::query()
+                ->where('customer_id', $customer->id)
+                ->where('status', 'pending')
+                ->where('type', 'upgrade')
+                ->delete();
+
+            if ($replaced > 0) {
+                $this->dispatch('notify', message: __('billing.cart.plan_replaced'));
+            }
+        }
+
         Order::create([
             'customer_id' => $customer->id,
             'plan' => $plan,
diff --git a/app/Models/Order.php b/app/Models/Order.php
index 5cb03a8..35fb018 100644
--- a/app/Models/Order.php
+++ b/app/Models/Order.php
@@ -43,6 +43,28 @@ class Order extends Model implements ProvisioningSubject
         };
     }
 
+    /**
+     * Monthly or one-off.
+     *
+     * Traffic is bought for the month that is running out and does not renew;
+     * everything else changes the recurring bill.
+     */
+    public function isRecurring(): bool
+    {
+        return $this->type !== 'traffic';
+    }
+
+    /** Net is what is stored; gross is what gets charged. */
+    public function grossCents(): int
+    {
+        return (int) round($this->amount_cents * (1 + self::taxRate()));
+    }
+
+    public static function taxRate(): float
+    {
+        return (float) config('provisioning.tax.rate_percent', 0) / 100;
+    }
+
     /** Only a pending order is still the customer's to change. */
     public function isRemovable(): bool
     {
diff --git a/config/provisioning.php b/config/provisioning.php
index 220c381..816fcb6 100644
--- a/config/provisioning.php
+++ b/config/provisioning.php
@@ -99,6 +99,16 @@ return [
         'sample_minutes' => 15,
     ],
 
+    /*
+    | Prices in this config are NET. The rate is configurable because it follows
+    | the seller's country (20 % in Austria), and every price shown to a
+    | customer has to say which it is — a figure without "netto" or "brutto" is
+    | the single most common billing complaint there is.
+    */
+    'tax' => [
+        'rate_percent' => (float) env('CLUPILOT_TAX_PERCENT', 20),
+    ],
+
     // Extra storage add-on (per unit) and the add-on catalogue (labels in lang/*/billing.php).
     'storage_addon' => ['gb' => 100, 'price_cents' => 1000],
     'addons' => [
diff --git a/lang/de/billing.php b/lang/de/billing.php
index 8ec321f..0def38c 100644
--- a/lang/de/billing.php
+++ b/lang/de/billing.php
@@ -1,7 +1,18 @@
 <?php
 
 return [
+    'net_per_month' => 'netto pro Monat',
+    'net_once' => 'netto, einmalig',
+    'net_hint' => 'netto, zzgl. :percent % USt.',
     'cart' => [
+        'net' => 'netto',
+        'per_month' => 'pro Monat',
+        'once' => 'einmalig',
+        'subtotal_net' => 'Zwischensumme netto',
+        'vat' => 'zzgl. :percent % USt.',
+        'total_gross' => 'Gesamt brutto',
+        'recurring_note' => 'Davon :amount brutto monatlich wiederkehrend.',
+        'plan_replaced' => 'Der bisher vorgemerkte Paketwechsel wurde ersetzt — es kann nur einer offen sein.',
         'title' => 'Vorgemerkte Käufe',
         'subtitle' => 'Wird nach Zahlung aktiviert',
         'added' => 'vorgemerkt am :when',
diff --git a/lang/en/billing.php b/lang/en/billing.php
index a4a1788..f2476a9 100644
--- a/lang/en/billing.php
+++ b/lang/en/billing.php
@@ -1,7 +1,18 @@
 <?php
 
 return [
+    'net_per_month' => 'net per month',
+    'net_once' => 'net, one-off',
+    'net_hint' => 'net, plus :percent % VAT',
     'cart' => [
+        'net' => 'net',
+        'per_month' => 'per month',
+        'once' => 'one-off',
+        'subtotal_net' => 'Subtotal net',
+        'vat' => 'plus :percent % VAT',
+        'total_gross' => 'Total gross',
+        'recurring_note' => 'Of which :amount gross recurs monthly.',
+        'plan_replaced' => 'The plan change you had pending was replaced — only one can be open at a time.',
         'title' => 'Pending purchases',
         'subtitle' => 'Activated once paid',
         'added' => 'added :when',
diff --git a/resources/views/livewire/billing.blade.php b/resources/views/livewire/billing.blade.php
index cb00d7a..129bc45 100644
--- a/resources/views/livewire/billing.blade.php
+++ b/resources/views/livewire/billing.blade.php
@@ -16,7 +16,7 @@
             </div>
             <div class="text-right">
                 <p class="text-2xl font-semibold text-ink">{{ $eur($current['price_cents'] ?? 0) }}</p>
-                <p class="text-xs text-muted">{{ __('billing.per_month') }}</p>
+                <p class="text-xs text-muted">{{ __('billing.net_per_month') }}</p>
             </div>
         </div>
         <div class="grid grid-cols-2 gap-px bg-line sm:grid-cols-4">
@@ -53,7 +53,13 @@
                                 {{ __('billing.cart.added', ['when' => $order->created_at->isoFormat('LL')]) }}
                             </p>
                         </div>
-                        <p class="font-mono text-sm text-body">{{ $eur($order->amount_cents) }}</p>
+                        <div class="text-right">
+                            <p class="font-mono text-sm text-body">{{ $eur($order->amount_cents) }}</p>
+                            <p class="text-xs text-faint">
+                                {{ __('billing.cart.net') }} ·
+                                {{ $order->isRecurring() ? __('billing.cart.per_month') : __('billing.cart.once') }}
+                            </p>
+                        </div>
                         <button type="button" aria-label="{{ __('billing.cart.remove') }}" title="{{ __('billing.cart.remove') }}"
                                 x-on:click="$dispatch('openModal', { component: 'confirm-remove-order', arguments: { uuid: '{{ $order->uuid }}' } })"
                                 class="grid size-8 place-items-center rounded-md border border-line text-muted transition hover:border-danger hover:text-danger">
@@ -63,9 +69,31 @@
                 @endforeach
             </ul>
 
-            <div class="flex items-center justify-between border-t border-line bg-surface-2 px-5 py-3">
-                <span class="text-sm font-medium text-body">{{ __('billing.cart.total') }}</span>
-                <span class="font-mono text-sm font-semibold text-ink">{{ $eur($pending->sum('amount_cents')) }}</span>
+            @php
+                $net = $pending->sum('amount_cents');
+                $gross = $pending->sum(fn ($o) => $o->grossCents());
+                $recurringNet = $pending->filter->isRecurring()->sum('amount_cents');
+            @endphp
+            <div class="space-y-1.5 border-t border-line bg-surface-2 px-5 py-3.5 text-sm">
+                <div class="flex items-center justify-between text-muted">
+                    <span>{{ __('billing.cart.subtotal_net') }}</span>
+                    <span class="font-mono">{{ $eur($net) }}</span>
+                </div>
+                <div class="flex items-center justify-between text-muted">
+                    <span>{{ __('billing.cart.vat', ['percent' => rtrim(rtrim(number_format(config('provisioning.tax.rate_percent', 0), 1, ',', '.'), '0'), ',')]) }}</span>
+                    <span class="font-mono">{{ $eur($gross - $net) }}</span>
+                </div>
+                <div class="flex items-center justify-between border-t border-line pt-1.5 font-semibold text-ink">
+                    <span>{{ __('billing.cart.total_gross') }}</span>
+                    <span class="font-mono">{{ $eur($gross) }}</span>
+                </div>
+                @if ($recurringNet > 0)
+                    {{-- The number that actually matters every month, kept apart
+                         from a one-off traffic top-up in the same cart. --}}
+                    <p class="pt-1 text-xs text-faint">
+                        {{ __('billing.cart.recurring_note', ['amount' => $eur((int) round($recurringNet * (1 + \App\Models\Order::taxRate())))]) }}
+                    </p>
+                @endif
             </div>
         </div>
     @endif
@@ -92,6 +120,7 @@
                             @endforeach
                         </ul>
                         <p class="mt-4 text-xl font-semibold text-ink">{{ $eur($p['price_cents']) }}<span class="text-sm font-normal text-muted"> / {{ __('billing.month_short') }}</span></p>
+                        <p class="text-xs text-faint">{{ __('billing.net_hint', ['percent' => rtrim(rtrim(number_format(config('provisioning.tax.rate_percent', 0), 1, ',', '.'), '0'), ',')]) }}</p>
                         <x-ui.button variant="primary" size="sm" class="mt-4 w-full" wire:click="purchase('upgrade', '{{ $key }}')" wire:target="purchase('upgrade', '{{ $key }}')" wire:loading.attr="disabled">
                             {{ __('billing.upgrade_cta') }}
                         </x-ui.button>
@@ -110,6 +139,7 @@
                 <p class="font-semibold text-ink">{{ __('billing.storage_title') }}</p>
             </div>
             <p class="mt-2 text-sm text-body">{{ __('billing.storage_body', ['gb' => $storage['gb'], 'price' => $eur($storage['price_cents'])]) }}</p>
+            <p class="mt-1 text-xs text-faint">{{ __('billing.net_per_month') }}</p>
             <x-ui.button variant="primary" size="sm" class="mt-4 w-full" wire:click="purchase('storage')" wire:target="purchase('storage')" wire:loading.attr="disabled">
                 {{ __('billing.storage_cta', ['gb' => $storage['gb']]) }}
             </x-ui.button>
@@ -122,6 +152,7 @@
                 <p class="font-semibold text-ink">{{ __('billing.traffic_title') }}</p>
             </div>
             <p class="mt-2 text-sm text-body">{{ __('billing.traffic_body', ['gb' => $trafficAddon['gb'], 'price' => $eur($trafficAddon['price_cents'])]) }}</p>
+            <p class="mt-1 text-xs text-faint">{{ __('billing.net_once') }}</p>
             @if ($trafficMeter)
                 <p class="mt-2 font-mono text-xs text-muted">
                     {{ __('billing.traffic_used', [
@@ -145,6 +176,7 @@
                 </div>
                 <p class="mt-2 flex-1 text-sm text-muted">{{ __('billing.addon.'.$key.'.desc') }}</p>
                 <p class="mt-3 text-lg font-semibold text-ink">{{ $eur($addon['price_cents']) }}<span class="text-sm font-normal text-muted"> / {{ __('billing.month_short') }}</span></p>
+                        <p class="text-xs text-faint">{{ __('billing.net_hint', ['percent' => rtrim(rtrim(number_format(config('provisioning.tax.rate_percent', 0), 1, ',', '.'), '0'), ',')]) }}</p>
                 <x-ui.button variant="secondary" size="sm" class="mt-3 w-full" wire:click="purchase('addon', '{{ $key }}')" wire:target="purchase('addon', '{{ $key }}')" wire:loading.attr="disabled">
                     {{ __('billing.addon_cta') }}
                 </x-ui.button>
diff --git a/tests/Feature/CartTest.php b/tests/Feature/CartTest.php
index da968f5..c5471e6 100644
--- a/tests/Feature/CartTest.php
+++ b/tests/Feature/CartTest.php
@@ -70,3 +70,65 @@ it('refuses to remove an order that is already paid', function () {
 
     expect(Order::query()->whereKey($order->id)->exists())->toBeTrue();
 });
+
+it('keeps at most one plan change in the cart', function () {
+    [$customer, $user] = cartCustomer();
+    App\Models\Instance::factory()->for($customer)->create(['plan' => 'start', 'status' => 'active']);
+
+    $component = Livewire\Livewire::actingAs($user)->test(Billing::class);
+    $component->call('purchase', 'upgrade', 'business');
+    $component->call('purchase', 'upgrade', 'enterprise');
+
+    // Two contradicting plan changes cannot both be paid for — the second
+    // replaces the first instead of stacking.
+    $upgrades = Order::query()->where('type', 'upgrade')->where('status', 'pending')->get();
+    expect($upgrades)->toHaveCount(1)
+        ->and($upgrades->first()->plan)->toBe('enterprise');
+});
+
+it('still lets add-ons stack', function () {
+    [$customer, $user] = cartCustomer();
+    App\Models\Instance::factory()->for($customer)->create(['plan' => 'team', 'status' => 'active']);
+
+    $component = Livewire\Livewire::actingAs($user)->test(Billing::class);
+    $component->call('purchase', 'storage');
+    $component->call('purchase', 'storage');
+    $component->call('purchase', 'traffic');
+
+    // Buying 200 GB as two packs is a perfectly sensible thing to want.
+    expect(Order::query()->where('type', 'storage')->count())->toBe(2)
+        ->and(Order::query()->where('type', 'traffic')->count())->toBe(1);
+});
+
+it('says net, gross and how often for every price', function () {
+    [$customer, $user] = cartCustomer();
+    config()->set('provisioning.tax.rate_percent', 20);
+
+    Order::factory()->for($customer)->create(['type' => 'storage', 'amount_cents' => 1000, 'status' => 'pending']);
+    Order::factory()->for($customer)->create(['type' => 'traffic', 'amount_cents' => 500, 'status' => 'pending']);
+
+    Livewire\Livewire::actingAs($user)->test(Billing::class)
+        ->assertSee(__('billing.cart.net'))
+        ->assertSee(__('billing.cart.per_month'))     // storage recurs
+        ->assertSee(__('billing.cart.once'))          // traffic does not
+        ->assertSee('12,00')                          // 15,00 net → 18,00 gross
+        ->assertSee('18,00')
+        ->assertSee(__('billing.cart.total_gross'));
+});
+
+it('separates what recurs from a one-off top-up', function () {
+    [$customer, $user] = cartCustomer();
+    config()->set('provisioning.tax.rate_percent', 20);
+
+    Order::factory()->for($customer)->create(['type' => 'storage', 'amount_cents' => 1000, 'status' => 'pending']);
+    Order::factory()->for($customer)->create(['type' => 'traffic', 'amount_cents' => 500, 'status' => 'pending']);
+
+    // 10,00 net recurring → 12,00 gross per month, not the 18,00 total.
+    // Formatted through the same helper the view uses, or the non-breaking
+    // space in the currency makes this a test about whitespace.
+    $expected = __('billing.cart.recurring_note', [
+        'amount' => Illuminate\Support\Number::currency(12.00, in: 'EUR', locale: 'de'),
+    ]);
+
+    Livewire\Livewire::actingAs($user)->test(Billing::class)->assertSee($expected);
+});