... and the way I understand these "stamps" is that if you use them you cannot lodge mail into a red street box.

It all has to go over the counter along with the presentation of your card.

I guess if you put one into a red street box you will cop an unpaid postage fine, or your recipient will.

That's what stops people from buying one of these stamps from China for $2 and dropping all the stamped mail into a red street box.