In Australia, most postal packaging products are commonly available in the retail shopping part of the Post Office premises, as single items.
You buy the exact number of them that you might need, be they envelopes, mailer bubble packs, mailer boxes or whatever.
The Royal Australian Mint also uses the Australian Post Office as an agency to sell their products. Selected Mint products are available in the retail shopping part of the Post Office. You can also apply for passports. Basically the APO is a retail outlet for other Australian Government agencies.
However I DON'T think that you could get a job application for ASIO* employment processed there!

*= Australian Security Intelligence Organisation. (similar to the CIA)
Doesn't the U.S. Post office system have a retail section? If they don't, they certainly should have to help boost sales, and get them out of the financial strife that they are in!
You can buy all sorts of stationery and computer supplies, books, DVD's and associated items in APO outlets. Oh, yes! You can also buy stamps, both for mailing AND collector issues.