This is a fantastic post & I love mail art and experimenting with the mail. One thing I learned in the USA is that while a card can be 3x5 (LxW) it cannot be 5x3 (i.e. oriented in "portrait") because the machines aren't set up to read it that way. However, you can add $.20 in additional postage and write "No machine" so they hand-cancel the stamp rather than using the machine. This is useful for any time you don't want them running the letter through the cancelling machine.

At the risk of opening myself up to having postal workers kick in my door and charge me with a federal crime: one experiment I tried was to stamp a postcard addressed to me, cover the address with a card with my friends address on it, shrink-wrapping it and sending it to them. The stamp does not get canceled because of the plastic wrap & the reply can travel back to me on the same stamp. (basically: "yes you can cover a stamp with plastic to avoid cancellation then reuse it.")

One nit I'd pick with the author here is that the address style he uses does not adequately route the letter to him. He states that all the mail in his building goes into one box, so if this were not something he was expecting and it arrived for him, there would be no way to route it to the correct person once it reached the building. I suppose in the more general case, assuming a single family household, this method would work. Cool post in any event!