When you sign up for Reddit Gifts, you are given 1 credit. When you sign up for an exchange, it costs 1 credit. When you have completed everything required in an exchange, you will receive your credit back, as well as an additional credit for completing the exchange. If you are a rematch Santa, you can register to give again and will receive a maximum of 1 bonus credit per exchange (not per rematch).
To see how many exchange credits you have, log in to Reddit Gifts and go to this link where your credits and a link to a breakdown of the credits are listed at the top of the page.
We have a credit system in place to try to ensure that people do not sign up for many exchanges just to receive gifts. If you fail to successfully complete an exchange, your credits will be reset to -1 until you complete all required steps of the incomplete exchange. Once you have completed the exchange, all credits will be restored.