[AnthonyMouse]

The post office doesn't need to know "who you are" except insofar as you have a payment method that works which they can charge postage to.

The payment processor, in turn, just needs to know that you're authorized to draw on that account, which they know because you have the credentials established when it was opened.

Functionally none of this actually uses your name for anything useful. Even giving it to them at all is, at best, a password reset method, and there are a million other ways to do that which don't require a centralized ID.