Firstly set up the item in your 'Manage My Products' list such as "Applied $50 Gift Voucher" allows you and your client to see the Voucher has been applied in their invoice/receipt.
Once the above has been set up, you'll be able to chose that "Applied $50 Gift Voucher" from the 'My Products' tab to add to the Clients cart/bill.
Then tap on the 'Apply Discount' tab at the top of the Clients Cart to then minus that Gift Voucher amount.
Watch this 1 min walkthrough:
*If you need further assistance, please reach out via the 'Get Help' function in your app!