I think we all make this mistake at some point when we are starting out. I remember booking an AA fare to SFO soon after I discovered the wonders of the FFP, only to find that I had done so in a non-earning class. That was the first and last time that ever happened to me. Now if I can't see the booking class before I hit purchase, I book somewhere that will tell me. In your case AC.com is one of the better sites for displaying the booking class prior to purchase.