"How do you get these?

And what are the benefits, if any?"

You have to have an account with the Aust Post.

You buy the rubber stamp from them.

EVERYthing you post under that account has to be stamped with the rubber stamp.

No more paper stamps, no $$$ amount on the envelopes or boxes.

(You can often make a $ or two on the postage).

Only one piece od paper to be filled in which receipt is stapled to so easy to find a record.

That's just my HO.