I needed 1 envelope last month. It was 1 PLN (0.3 USD) at nearest street shop (kiosk). I don't think they lose money on that. I also bought a pen there for another 1 PLN.

They mostly sell cigarettes, newspapers, chewing gum and tickets, but they keep some envelopes, empty DVDs, pens, lighters, etc - it's not going to expire and they have huge markup, so why not?

Are there no small shops in your country?