feat(paypal): add rounding adjustment to fix amount mismatch

Introduce a new method `fixPaypalAmountMismatch` to handle edge cases where
the calculated total does not match the expected grand total due to floating
point precision issues. This ensures accurate totals are sent to the PayPal API.

The method adjusts either the discount total or adds a rounding adjustment item
to reconcile any discrepancies.

Author: hirale

app/code/core/Mage/Paypal/Model/Order.php

@@ -191,6 +191,10 @@ public function buildPurchaseUnit(Mage_Sales_Model_Quote $quote, ?string $refere
         $totals = $adjustedCartData['totals'];
         $items  = $adjustedCartData['items'];
 
+        $fixedCartData = $this->fixPaypalAmountMismatch($cart, $currency);
+        $totals = $fixedCartData['totals'];
+        $items  = $fixedCartData['items'];
+
         $breakdown = $this->buildAmountBreakdown($totals);
         $amount    = $this->buildAmountWithBreakdown($currency, $quote->getGrandTotal(), $breakdown);
 
@@ -347,6 +351,61 @@ public function adjustCartTotalsForRounding(Mage_Paypal_Model_Cart $cart, string
         return ['totals' => $totals, 'items' => $items];
     }
 
+    /**
+     * Fix amount mismatch edge cases for PayPal API by adding rounding adjustments
+     *
+     * @return array{totals: array, items: array}
+     */
+    public function fixPaypalAmountMismatch(Mage_Paypal_Model_Cart $cart, string $currency): array
+    {
+        $totals = $cart->getAmounts();
+        $items  = $cart->getAllItems();
+
+        $itemTotal = isset($totals[Mage_Paypal_Model_Cart::TOTAL_SUBTOTAL])
+            ? (float) $totals[Mage_Paypal_Model_Cart::TOTAL_SUBTOTAL]->getValue()
+            : 0.00;
+
+        $taxTotal = isset($totals[Mage_Paypal_Model_Cart::TOTAL_TAX])
+            ? (float) $totals[Mage_Paypal_Model_Cart::TOTAL_TAX]->getValue()
+            : 0.00;
+
+        $shippingTotal = isset($totals[Mage_Paypal_Model_Cart::TOTAL_SHIPPING])
+            ? (float) $totals[Mage_Paypal_Model_Cart::TOTAL_SHIPPING]->getValue()
+            : 0.00;
+
+        $discountTotal = isset($totals[Mage_Paypal_Model_Cart::TOTAL_DISCOUNT])
+            ? (float) $totals[Mage_Paypal_Model_Cart::TOTAL_DISCOUNT]->getValue()
+            : 0.00;
+
+        $expectedTotal = round($itemTotal + $taxTotal + $shippingTotal - $discountTotal, 2);
+        $grandTotal = (float) $cart->getQuote()->getGrandTotal();
+        $difference = round($expectedTotal - $grandTotal, 2);
+
+        if (abs($difference) > 0) {
+            if ($difference < 0) {
+                $newDiscountTotal = round($discountTotal + abs($difference), 2);
+                $totals[Mage_Paypal_Model_Cart::TOTAL_DISCOUNT] = MoneyBuilder::init(
+                    $currency,
+                    number_format($newDiscountTotal, 2, '.', ''),
+                )->build();
+            } else {
+                $moneyBuilder = MoneyBuilder::init($currency, number_format($difference, 2, '.', ''))->build();
+                $roundingItem = ItemBuilder::init(
+                    Mage::helper('paypal')->__('Rounding Adjustment'),
+                    $moneyBuilder,
+                    quantity: '1',
+                )
+                    ->sku(Mage::helper('paypal')->__('rounding_fix'))
+                    ->category(ItemCategory::DIGITAL_GOODS)
+                    ->build();
+
+                $items[] = $roundingItem;
+            }
+        }
+
+        return ['totals' => $totals, 'items' => $items];
+    }
+
     /**
      * Update payment object with PayPal order information
      */