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Themes/Storefront/resources/lang/en/invoice.php

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+<?php
+
+return [
+    'subject' => 'Invoice for order #:id',
+    'invoice' => 'INVOICE',
+    'order_id' => 'Order ID',
+    'date' => 'Date',
+    'order_details' => 'Order Details',
+    'email' => 'Email',
+    'phone' => 'Phone',
+    'payment_method' => 'Payment Method',
+    'billing_address' => 'Billing Address',
+    'shipping_address' => 'Shipping Address',
+    'product' => 'Product',
+    'unit_price' => 'Unit Price',
+    'quantity' => 'Quantity',
+    'line_total' => 'Line Total',
+    'subtotal' => 'Subtotal',
+    'coupon' => 'Coupon',
+    'total' => 'Total',
+];