Ah, that's a little more involved than what I'm used to in the United States.

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.