Does anyone know if you can order packages off Amazon on post in Germany?

You can, it won't be delivered by DHL as you stated in your other comment though. It would get delivered to the sorting facility in New York and be shipped over like most other mail. Just remember it won't be at the normal amazon speed. It still might take a week or two to arrive. The biggest issue though is when the stuff you order goes through New Jersey, that means generally its coming by boat and will take a minimum of 6 weeks.

Yes but it will take time. After all you are shipping to an APO Address unless you live off base with local mailbox

Not all shipments from amazon.com will deliver to an APO address, which is what you’ll have, but amazon.de is amazing as well if you have a friend or if you yourself will have an off post address ie married.
Any other questions, PM me!