It's a Taco Shop

On 5/31/2012, my boyfriend and I went to It's a Taco Shop's drive thru. We ordered 4 items and the cashier gave us our total of $19.80; we paid with a Visa. My boyfriend signed the receipt and noticed that he was charged $21.30. The receipt had our ordered items listed, and at the bottom, there was a charge for $1.50 - it had been added as "x onion," but when the receipt printed, the cashier had crossed out "x onion" and written the word "fee." I did not see a sign stating that using a Visa, or any credit card for that matter, would incur an addition fee of $1.50. Furthermore, it is my belief that it is against the law to charge the customer a fee when using their credit card. I can provide the receipt information if needed. Thank you.