diff --git a/src/Core/OrderRequestFactory.php b/src/Core/OrderRequestFactory.php
index df333995..dc46afb5 100644
--- a/src/Core/OrderRequestFactory.php
+++ b/src/Core/OrderRequestFactory.php
@@ -206,7 +206,7 @@ protected function getAmount(): AmountWithBreakdown
 
         // possible price surcharge
         if ($discount < 0) {
-            $itemTotal += $discount;
+            $itemTotal -= $discount;
             $discount = 0;
         }
         $total = $itemTotal - $discount;