I moved earlier this summer and my new closest PO has an APC machine (not sure why they are so scarce on the ground in this area). I recall that there is some way to send media mail from an APC but that it's a bit of a workaround. Can anyone remind me of the process? Thanks.

Instead of selecting a specific type of mail class, all you have to do is choose the option where you enter your own postage value. It's very easy and not too many steps. In fact, it's been a month or two since I've done it and I think its the first or second screen/option. You just need to know the media mail postage increments and how much your package weighs to enter the amount you need.

Weigh your package at home and enter the correct weight in PBS when you generate the label. That will tell you what the postage should be. At the APC, select "buy stamps" then the button that says something like "buy a stamp of another value". Enter the postage from the PBS label. It will ask how many you want. Pay for your postage, and stick the stamp that comes out of the APC onto your package.

If you don't have a postal scale at home, you can print the current media mail chart here and take it with you to the Post Office.

Print your label as usual, then go to the APC machine.  Let the machine weigh your package then use the chart and choose the option to choose the 'stamp' amount.  Don't forget to add .23 for Tracking if you have printed your label that way.
 

Just don't forget to write "MEDIA MAIL" on your package :) 

Thanks very much, all!