... why not try Read it swap it? Register your books, and if you like someone else's, you can offer to swap for one of theirs. You only pay for the postage.
Incidentally, it would definitely be morally wrong to put your own swapping parcels in your employer's post tray. Even if said employer was a library and therefore parcels of books in the post would raise no eyebrows.
Bookcrossing is an infinite, random book exchange -- you register your books then release them into the wild -- leave them on a bench, give them to a charity shop, leave them on a bus -- and eventually the person who finds it should post on the bookcrossing website to tell you where they picked it up.