I've never used one so someone else can explain better, but my understanding is that you get an envelope and you can use your phone to pre-pay postage online (or maybe set up your card to be charged based on what's required?) and you'll get a 9 digit code back. On the corner of all the envelopes is a 3x3 grid where you write the code and that is read by the postal system as the stamp.
You buy a code that you write down in the location where normally the stamp would be.
Just for fun, seems easily susceptible to a man-in-the-middle attack. Just take a postcard, copy their code, discard the original card, or just send the original later than the copy. :)
Yes, but then you are stealing mail. Which is bad. For a relatively small payoff.
stealing mail is also punishable with jail time basically everywhere. they take that stuff seriously