It's awesome that you're able to offer your customers gift certificates! I know they can be a bit tricky to figure out for recording purposes since QuickBooks Online doesn't have a gift certificate button to set these up or record them. Not to worry. Even though there isn't a button for this, there is still a way to record gift certificates that you're giving to your customers.
Despite not having a button to do so, selling and redeeming gift certificates or gift cards in QuickBooks Online is still a fairly straightforward and simple process. In fact, the QuickBooks community has a help article just for this sort of thing, which I encourage you to review here: Sell and redeem gift cards or certificates in QuickBooks Online The article goes over how to sell a gift certificate, how to redeem one, and what to do is the gift certificate has more funds than are being used on a single purchase.
Feel free to bookmark that article. I think it'll be handy for you moving forward.
Don't be afraid to reach out with more questions! I'm here to help. :)