You can make an effort of good faith by purchasing a money order or a certified bank check addressed to the creditor for the full amount you agreed to pay. (creditrepair.org)
You can't send cash in the mail, so purchase a money order for any cash deposits and mail the money order. (sapling.com)
For instance, some people have found grocery - store clerks who allow them to purchase money orders on credit cards. (creditcards.com)