Private Post (Privatpost) is an interesting collecting category. In many locations mail was not delivered to the recipient's address, but was held at the local post office for collection. For those who were either too busy or couldn't be bothered doing this, private companies were set up to collect mail for subscribers and deliver it to their address, for a fee. Most of them produced their own stamps.

Available here is the following Private Post stamp:

[STETTIN, 1895] COURIER. PRIVATSTADTBRF. BEFOERDERUNG 3 PFENNIG