How do I set a tax % for a given transaction? Add items to your cart. 2. Use the Tap to Proceed button to be directed to the Review Order screen. 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.