I wrote them this to them:
Yesterday, I successfully applied a $10 credit to my account of which I made a screen print of the terms and conditions and the promo active upon checkout. Today it is no longer in my cart. It was the Get $10 of at Amazon for using Mastercard. The terms said "You may redeem your credit by purchasing eligible product(s) from Amazon by 11:59 p.m. PT on January 14, 2017. UNREEDEMED CREDIT WILL EXPIRE AFTER THAT TIME," leading anyone to believe that you have until next year to use the credit after getting it applied. It seems deliberately misleading and I never expected it to be revoked based on this language. I expected to fulfill my order before that date.
Amazon.com like many companies play with this language to maximize deception with the fallback language of that it can be canceled or modified at anytime.