How do I set a tax % for a given transaction? Add items to your cart. Phone: Tablet: Tap Subtotal to go to the Review Order screen: Phone: Tablet: Tap the Tax row to customize the tax for this specific transaction. You will be prompted with the following screen. Input the percentage, and tap the check-mark in the upper right-hand corner to save. Tap Select Tender to proceed with a transaction. For additional support, please refer to the Support Page in your merchant portal or contact your ISO contact.