Ugh. I hate credit cards. I agree--it will be nice when they are all paid off. I had a somewhat similar situation with a credit card that I got when I was a freshman in college (free tshirt and chocolate--sure I'll sign up!). I had stopped using the card and it had no balance. The company sent me a statement and a letter saying they had given me a $20 credit that I had 30 days to use or it would lapse. So I put $20 worth of gas in my car (back when that would actually almost fill your tank). When I got my next statement, I expected to see a $20 credit with a $20 debit and thus a zero $0 owed. Nope! They did the double-billing cycle thing and charged me 22% interest on the $20, and then at the very end of the billing cycle, they subtracted the $20 credit but I was left with an interest payment! Now, it wasn't very much but I had to PAY to use a $20 credit on a $20 purchase! Now, it was nice to have paid only cents for gas in the car but I had to send a check in for pennies to pay the interest. The next time the offer came, I let it lapse.
If I wouldn't have checked my statement carefully (thinking I owed $0), I wouldn't have sent in a check for the interest, would have had a $29 late fee (or more), my credit score dropped, 22% (or more) interest charged on the interest, etc.
I was almost bitten. When it is "too good to be true" from a credit card company, be warned!