Some envelopes aren't stamped but are still sent. How?

That ink square in the corner is the ‘stamp’. It is actually called metered mail because the machine used to place the mark is a meter. Generally you buy postage for a meter in advance, the meter deducts the amount of postage from its preset limit as it marks the envelope.

When the postage on a meter runs out you can buy more from the post office with just a phone call. Most of the meters are now linked by phone or internet to the PO.

It can also be bulk mail, and the mailer puts a bulk mail stamp on the envelope and then the mail is weighted and paid for at the time it is mailed.

What @WestRiverrat is describing is “franking”.

Franking can also be preprinted on the envelops.

@Lightlyseared Technically you are correct. Our new post master calls it metering, so we use the term so he doesn’t get confused.

@WestRiverrat has described the metered mail process correctly.

I used to work at a bank running the bank’s mail through a mail machine that would print the ink square you’re talking about. We used a company called Pitney Bowes. Also the info printed within the ink square would usually contain the date, zip code and name of machine’s company. A mail machine does all the work, usually this is done for bulk mail, but non bulk mail can be processed as well.