USPS website won’t accept my password.

We're not the US Postal System USPS, nor folks that work for Apple, and we don't have access to your USPS nor your Apple ID login info.

If you have questions or issues with the USPS website or with your login there, you will need to discuss those with the USPS support folks.

Here is the USPS forgotten-password webpage: https://reg.usps.com/entreg/ForgotPasswordAction_input

Usual spot for storing passwords is in Keychain, though details of that depend on whether you're using iPhone or iPad, or Mac, and whether you're using iCloud Keychain. Storing passwords with your Contacts info is less common, and not particularly secure.

If you have a USPS password that you can't remember you will have to contact USPS to reset that password. Your USPS password has nothing to do with Apple.

Sounds like a USPS web site question.

Have you contacted their tech support.?

https://usps.force.com/emailus/s/informed-delivery-inquiry