diff --git a/app/Livewire/Billing.php b/app/Livewire/Billing.php
index f9fc2a5..bc57f3c 100644
--- a/app/Livewire/Billing.php
+++ b/app/Livewire/Billing.php
@@ -3,7 +3,9 @@
 namespace App\Livewire;
 
 use App\Livewire\Concerns\ResolvesCustomer;
+use App\Models\Customer;
 use App\Models\Order;
+use Illuminate\Support\Facades\DB;
 use Livewire\Attributes\Layout;
 use Livewire\Component;
 
@@ -54,32 +56,42 @@ 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();
+        // A cart cannot hold two plan changes: they contradict each other and no
+        // checkout could resolve which one the customer meant. Choosing another
+        // replaces the pending one — add-ons still stack.
+        //
+        // Replacement and insert run together with the customer row locked:
+        // two clicks in flight could otherwise both delete and then both
+        // insert, leaving exactly the two upgrades this rule exists to prevent.
+        $replaced = DB::transaction(function () use ($customer, $type, $plan, $addonKey, $amount, $datacenter) {
+            Customer::query()->whereKey($customer->id)->lockForUpdate()->first();
 
-            if ($replaced > 0) {
-                $this->dispatch('notify', message: __('billing.cart.plan_replaced'));
-            }
+            $replaced = $type === 'upgrade'
+                ? Order::query()
+                    ->where('customer_id', $customer->id)
+                    ->where('status', 'pending')
+                    ->where('type', 'upgrade')
+                    ->delete()
+                : 0;
+
+            Order::create([
+                'customer_id' => $customer->id,
+                'plan' => $plan,
+                'type' => $type,
+                'addon_key' => $addonKey,
+                'amount_cents' => $amount,
+                'currency' => 'EUR',
+                'datacenter' => $datacenter,
+                'status' => 'pending',
+            ]);
+
+            return $replaced;
+        });
+
+        if ($replaced > 0) {
+            $this->dispatch('notify', message: __('billing.cart.plan_replaced'));
         }
 
-        Order::create([
-            'customer_id' => $customer->id,
-            'plan' => $plan,
-            'type' => $type,
-            'addon_key' => $addonKey,
-            'amount_cents' => $amount,
-            'currency' => 'EUR',
-            'datacenter' => $datacenter,
-            'status' => 'pending',
-        ]);
-
         $this->dispatch('notify', message: __('billing.purchased'));
     }