A means of payment through the post office is

A means of payment through the post office is a "money order." A money order is a payment order for a pre-specified amount of money, which is issued and guaranteed by the post office. It is a secure and reliable way to transfer funds, especially when you need to make payments to someone who doesn't accept personal checks or online payments. You can purchase a money order at your local post office, fill it out with the recipient's name and address, and mail it to them or hand it to them in person. The recipient can then cash the money order at their own post office or bank. Unlike personal checks, money orders do not bounce, and they can be replaced if lost or stolen (for a fee).