If I mail a check to my dr for payment, and enclose a letter saying "If you cash this check it means the bill is paid in full" and I send the letter certified return receipt with restricted delivery and above the signature box on the return receipt I write, "signature means you will accept and cash the enclosed check" is the doctor than legally obligated to abide by the agreement in the letter (that the check will be cashed and bill paid in full) if he signs the return receipt?