This is my GC:
http://www.etsy.com/listing/84180407/25-dollar-gift-certificateI put the code on when it's purchased, and write it in my spreadsheet.
Etsy doesn't allow you to do to dollar amount 'coupon codes' so the person has to select "other" as their payment option when checking out their cart.
(I dont have it enabled at the moment cuz I haven't sold one yet, but to enable "other" as a payment option you go into 'shipping & payments' => 'payment method').
Then in the message to seller, they put the code that is on their gift certificate.
With the "other" payment option, they don't pay anything so if there is a balance owing on the order (say they bought $35 worth of stuff including shipping but their GC was for $25, so they still owe you $10), then you just send them a paypal invoice for that $10.
Does my explaining make sense? lol