At least in Germany I usually get a QR code for the online bought postage I just show at the local post office on my phone. They print out the the postage thing on a sticker. I need to go there anyways with the package.
Here I can print a piece of paper with an address and 2D bar code on it (which is the postage), tape the label to a box, and leave it where the mailman can find it (usually on my front porch), drop it in the outgoing mail at work, or hand it to any mailman I see. No visiting a post office to mail things.