1. Enter a New Invoice for the issuance of a new Gift Certificate to be used by valid user (Customer).
2. Create a reversing Journal Entry for the newly created invoice in Step 1. Do not forget to enter the customer's name.
3. Go to customer record and click 'Accept Payment' button.
4. Apply the Journal Entry to the new invoice to make it paid in full.
5. Edit the Invoice where you would like to apply the Gift Certificate to as payment.
6. Under Billing tab > Payment sub tab > enter the Gift Certificate 'Authentication Code' created from Step 1. You can also check the Authentication Code by navigating to Lists > Accounting > Gift Certificates.
7. Click Save.
Result: Invoice should now have a status 'Paid In Full'.