I'm nearly positive that I earned one before.. but I only have one now when I thought I would have two. So do they expire, or am I wrong and I just didn't get one the last time I bought CSC?

No, they don't expire c: I believe they're given out with the stamps, for every $10 spent. So if you spent less than $10 you wouldn't have one. You do need to go to the Rewards Card page to claim your point(s) though