Como dizer "formulário de contestação" em inglês
The word "affidavit" in English means a duly sworn statement. Adding the word "letter" doesn't really express anything. You can send or submit affidavits to whomever it may concern, of course. If you lose a c/c in N America, the thing to do is to call the company as soon as possible so that they may block the card from further use, and, generally, they accept your statement of the facts and discharge you from responsibility for the fraudulent purchases. Time is of the essence, of course; notification should occur as promptly as possible.