M/C Surcharge at Retail Stores?

Has anyone noticed an unexplained surcharge on your receipt for using a MasterCard? I bought some stuff at Sports Closet (formerly the Oilers Store) and noticed they charged me $7 for using my MasterCard. What the heck!?!? I didn’t see any signs to advise me of this and I tend to be quite observant about those types of things. Maybe this is common practice now and I just haven’t noticed?