What's the word for the action of using a one-time special offer?
Solution 1:
Like you mention, I think redeem is the word you're looking for:
redeem
transitive verb To turn in (coupons, for example) and receive something in exchange.
transitive verb To convert into cash.
transitive verb To fulfill (a pledge, for example).
Depending on the context, you might also use "exchange," but it seems that "redeem" is your best option.