You cannot buy Media Mail postage labels online from usps.com, but you can buy it from paypal.com and pirateship.com
UPDATE: PayPal will be outsourcing their shipping service to ShipStation starting in July 2021. So, scroll down to the section about Pirate Ship, for now. You can buy media mail postage there. This article will be updated with ShipStation information as I write it. THX – JK.
Continue reading How to Buy Media Mail Postage Online (Hint: PayPal)