diff --git a/core/components/minishop3/src/Services/Order/ManagerOrderCostRecalculator.php b/core/components/minishop3/src/Services/Order/ManagerOrderCostRecalculator.php
index 4a5f2b9c..c4229f64 100644
--- a/core/components/minishop3/src/Services/Order/ManagerOrderCostRecalculator.php
+++ b/core/components/minishop3/src/Services/Order/ManagerOrderCostRecalculator.php
@@ -87,7 +87,7 @@ public function recalculate(msOrder $order, array $options = []): array
 
         $deliveryCost = $deliveryResult['delivery_cost'];
 
-        $paymentBase = round($cartCost + $deliveryCost, 6);
+        $paymentBase = OrderService::paymentCommissionBase($cartCost);
         $paymentResult = $this->resolvePaymentFee($order, $paymentBase, $mode, $options);
         if (!$paymentResult['success']) {
             return $paymentResult;
diff --git a/core/components/minishop3/src/Services/Order/OrderCostCalculator.php b/core/components/minishop3/src/Services/Order/OrderCostCalculator.php
index 7e557394..a1379991 100644
--- a/core/components/minishop3/src/Services/Order/OrderCostCalculator.php
+++ b/core/components/minishop3/src/Services/Order/OrderCostCalculator.php
@@ -186,13 +186,14 @@ public function getPaymentCost(?msOrder $draft, array $orderData, string $token,
             return $this->success('ms3_order_getcost_success', ['cost' => $paymentCost]);
         }
 
-        // Get cart cost for payment calculation
+        // Get cart cost for payment calculation (MS2-compatible base: cart only)
         $cartCostResponse = $this->getCartCost($draft, $token, $ctx);
         $cartCost = $cartCostResponse['success'] ? $cartCostResponse['data']['cost'] : 0;
+        $paymentBase = OrderService::paymentCommissionBase((float) $cartCost);
 
-        // Payment getCost returns total with payment fee, so subtract cart cost
-        $costWithPayment = $msPayment->getCost($draft, $cartCost);
-        $paymentCost = $costWithPayment - $cartCost;
+        // Payment getCost returns total with payment fee, so subtract base
+        $costWithPayment = $msPayment->getCost($draft, $paymentBase);
+        $paymentCost = $costWithPayment - $paymentBase;
 
         $response = $this->ms3->utils->invokeEvent('msOnGetPaymentCost', [
             'calculator' => $this,
diff --git a/core/components/minishop3/src/Services/Order/OrderService.php b/core/components/minishop3/src/Services/Order/OrderService.php
index cc722446..c538b992 100644
--- a/core/components/minishop3/src/Services/Order/OrderService.php
+++ b/core/components/minishop3/src/Services/Order/OrderService.php
@@ -26,6 +26,14 @@ public function __construct(modX $modx)
         $this->modx = $modx;
     }
 
+    /**
+     * Base amount for payment commission (MS2-compatible: cart cost only, delivery excluded).
+     */
+    public static function paymentCommissionBase(float $cartCost): float
+    {
+        return round($cartCost, 6);
+    }
+
     /**
      * Clamp computed order total so it never goes negative (defence-in-depth vs misconfigured discounts).
      *
diff --git a/core/components/minishop3/tests/PaymentCommissionBaseTest.php b/core/components/minishop3/tests/PaymentCommissionBaseTest.php
new file mode 100644
index 00000000..5422af7a
--- /dev/null
+++ b/core/components/minishop3/tests/PaymentCommissionBaseTest.php
@@ -0,0 +1,67 @@
+<?php
+
+/**
+ * Guards payment commission base (issue #372): MS2-compatible cart-only base.
+ *
+ * Run: php tests/PaymentCommissionBaseTest.php
+ */
+
+declare(strict_types=1);
+
+require __DIR__ . '/../vendor/autoload.php';
+
+use MiniShop3\Services\Order\OrderService;
+use MiniShop3\Utils\PriceAdjustment;
+
+$fail = static function (string $message): never {
+    fwrite(STDERR, "FAIL: {$message}\n");
+    exit(1);
+};
+
+$cartCost = 1000.0;
+$deliveryCost = 500.0;
+$percent = '3%';
+
+$base = OrderService::paymentCommissionBase($cartCost);
+if ($base !== 1000.0) {
+    $fail('paymentCommissionBase must round cart cost only');
+}
+
+$fee = PriceAdjustment::calculate($base, $percent);
+if (abs($fee - 30.0) > 0.000001) {
+    $fail('3% of cart 1000 must be 30, got ' . $fee);
+}
+
+$wrongFee = PriceAdjustment::calculate($cartCost + $deliveryCost, $percent);
+if (abs($wrongFee - 45.0) > 0.000001) {
+    $fail('sanity: 3% of cart+delivery 1500 must be 45');
+}
+
+if (abs($fee - $wrongFee) < 0.000001) {
+    $fail('cart-only and cart+delivery bases must differ for this repro');
+}
+
+$recalculatorSource = file_get_contents(__DIR__ . '/../src/Services/Order/ManagerOrderCostRecalculator.php');
+if ($recalculatorSource === false) {
+    $fail('cannot read ManagerOrderCostRecalculator.php');
+}
+
+if (!str_contains($recalculatorSource, 'OrderService::paymentCommissionBase($cartCost)')) {
+    $fail('ManagerOrderCostRecalculator must use OrderService::paymentCommissionBase($cartCost)');
+}
+
+if (preg_match('/paymentBase\s*=\s*round\s*\(\s*\$cartCost\s*\+\s*\$deliveryCost/s', $recalculatorSource) === 1) {
+    $fail('ManagerOrderCostRecalculator must not use cart+delivery as payment base');
+}
+
+$calculatorSource = file_get_contents(__DIR__ . '/../src/Services/Order/OrderCostCalculator.php');
+if ($calculatorSource === false) {
+    $fail('cannot read OrderCostCalculator.php');
+}
+
+if (!str_contains($calculatorSource, 'OrderService::paymentCommissionBase')) {
+    $fail('OrderCostCalculator must use OrderService::paymentCommissionBase');
+}
+
+fwrite(STDOUT, "OK PaymentCommissionBaseTest\n");
+exit(0);