It's been a while since I've used a gift certificate but if I remember correctly, in the right-hand column there is a place to put a code for a coupon and one for gift certificates. It then removes that amount from your total and you don't have to worry about payments if it's zero.

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Available on Amazon for the Kindle E-Reader Monster of the North and The Shimmering Mage

Today I break my own personal record for the number of days for being alive.
Check out my store here or my free stuff here
I use Poser 13 and win 10