If you have Gift Cards unlocked, as an Admin user, you can easily add a Gift Card record to a customer's account. Follow the steps below.
You can add the Gift Cards feature for an additional $7/month. Contact us via chat if you are interested.
On a customer's Billing page, click on "Manage" under "Gift Cards & Prepayments."
Select "New record" then "Gift card credit record."
On the next screen, enter the amount then click "Submit."
The Gift Card record will appear in the "Gift Cards & Prepayments" under "History."
See this support article on how to add a Gift Card to an Invoice.