Go to Sales, then Gift cards. Sell gift cards at the register, or let customers buy them on your site (the address is in Payouts and payment settings): we email the code to them, or to the person it's for. Gift cards pay for anything: sales at the register, invoices, and more. Each card's balance is tracked as it's used.
Gift cards never expire and have no fees. Where your state's law requires it, customers can ask for a small balance back in cash; your terms of sale say how.