(1) Your USPS-provided email should neither be mandatory nor the only acceptable email. It could be an extra convenience, and a low-friction way to get a reasonably secure email for first-time / technically unsavvy users.

(2) An entity you're directly paying to may go out of business, sometimes due to circumstances beyond their control. At least state-sponsored entities don't do it so abruptly in most of the "civilized world".