diff --git a/api/checkout.php b/api/checkout.php
index 0911d2b..864e2e9 100644
--- a/api/checkout.php
+++ b/api/checkout.php
@@ -46,10 +46,26 @@ try {
         $stmt_prefix->execute([$branch_id]);
         $prefix = $stmt_prefix->fetchColumn() ?: 'ORD';
         if (strlen($prefix) > 3) $prefix = substr($prefix, 0, 3);
-        $prefix = str_pad($prefix, 3, 'X'); // Just in case it's shorter than 3
+        $prefix = strtoupper(str_pad($prefix, 3, 'X'));
 
-        // Format order_number as XXX#####1 (5 digits for #####)
-        $order_number = strtoupper($prefix) . str_pad($order_id, 5, '0', STR_PAD_LEFT) . '1';
+        // Determine the next serial number for this branch
+        // We look at existing order numbers for this branch that follow the XXX-###### format
+        $stmt_max = $pdo->prepare("SELECT order_number FROM orders WHERE branch_id = ? AND order_number LIKE ? ORDER BY id DESC LIMIT 100");
+        $stmt_max->execute([$branch_id, "$prefix-%"]);
+        $existing_orders = $stmt_max->fetchAll(PDO::FETCH_COLUMN);
+        
+        $max_serial = 0;
+        foreach ($existing_orders as $onum) {
+            // Format is XXX-######.
+            if (preg_match('/' . preg_quote($prefix) . '-(\d{6})/', $onum, $matches)) {
+                $serial = (int)$matches[1];
+                if ($serial > $max_serial) $max_serial = $serial;
+            }
+        }
+        $next_serial = $max_serial + 1;
+
+        // Format order_number as XXX-###### (6 digits)
+        $order_number = $prefix . '-' . str_pad($next_serial, 6, '0', STR_PAD_LEFT);
         
         // Update the order with the generated order_number
         $stmt_update = $pdo->prepare("UPDATE orders SET order_number = ? WHERE id = ?");