Great advice all! The crazy part is that I signed the credit card receipt and did not even notice. I am usually a little more observant than that!
I tend to watch the register entries and compare that receipt but have never had a different amount on the actual credit card transaction before. Just goes to show me - boy am just glad I saved the receipt! (Truthfully, I probably caused the distraction that resulted in the error - I can be quite a Chatty Kathy at the register!)