Can I be charged more if I use a credit card?

No. Texas law states that a merchant, with the exception of a government agency, may not charge a customer more if the customer uses a debit card instead of cash. Of course, the law doesn't require that merchants take credit cards, but once they do they may not charge you more on average. I suggest you let the store manager know that he didn't have the right to charge you the extra 5%, and that if he doesn't refund it, you will take the store in question to court if needed. Charing this illegal extra amount on a card also violates a basic tenant of Deceptive Trade Practice Act, and you could be entitled to at least double the amount paid. You also may want to let the attorney general's office know if you notice this practice.