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 (o

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.