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.
Log into PayPal, and then go here https://www.paypal.com/shiplabel/createbulk
That’s also in the site navigation as “MultiOrder Shipping“.
The purchasing process is multi-step. You create an “order”, then you buy one or more labels for the order, then you print the labels.
It might seem complex, at first, but the software is designed to send big orders to multiple people. Instead of sending one-by-one, you send in batches.
You first define your destinations, and then packages, because there may be more than one package per destination, and then you pay for it all at once. Then you print each label.
You can buy all kinds of postage with this tool, not just Media Mail. However, you can buy that other postage on USPS.com. PayPal’s shipping tool just allows you more flexibility to print more types of labels.
Pirate Ship is one of a few websites that let you purchase USPS postage labels and get a discount for pre-printing labels. Pirate Ship supports Media Mail postage!
The one caveat is that Pirate Ship requires you to pre-pay for postage, on a credit card. You then use up the credit, and when you run out, it’ll charge you for the same amount.
For example, I have mine set to buy $50 of postage at a time. Since I send a fair number of parcels, it reorders more postage every few weeks.