The U.S. Postal Service is now accepting Apple Pay, however Apple's payment platform is only currently accepted online, and probably won't be usable for in-person transactions for quite a while.
The USPS hasn't been a supporter of Apple Pay, but recently, it has adopted Apple's mobile payments system for some transactions. An update to its website allows users to use Apple Pay to purchase various services, including through its assorted apps.
This means Apple Pay can be used in transactions for The Postal Store, PO Box Online, Every Door Direct Mail, Click-N-Ship, and USPS Tracking Plus among others, as spotted by Appleosophy. To make the Apple Pay purchase, users must be using Safari version 10.0 or later on a supported iPhone, iPad, or Mac.
The new payment method is progress for the USPS, and a change that will help people with their holiday packages. It's still not a total acceptance of Apple Pay yet. It only applies to the website, and not any over-the-counter transactions at a physical Post Office.
Such a change to enable in-person Apple Pay transactions would require a major upgrade in infrastructure and terminals, affecting some 34,000 locations across the United States.
However, USPS' willingness to use Apple Pay on the website could be a sign that you may be able to buy postage using Apple Pay at Post Offices in the not too distant future.