Could I have paid for the envelope with stamps?

Probably not. The envelope is part of that particular branch’s inventory, and it would have to be accounted for through their cash register system. Stamps aren’t used as currency, so it wouldn’t have worked. Although you bought the stamps from USPS (or Canada Post, whatever), that has no bearing on whether the office you visited gets reimbursed for their product.

No.

Actually, this is a great question.
Postage stamps have served as money in areas as diverse as America, Europe, and the Far East. During the American Civil War merchants, struggling with a shortage of small coins, began the practice of making small change with postage stamps. source

Good luck trying to exchange postage stamps for a bus ride these days though. There was a bit on The Office about reporting a bus driver who wouldn’t accept stamps as fare: “I’ll report him as I’m walking home”.

I asked the postal person a similar question once. He said, “We sell postage. We don’t accept it as currency.” The occasion was a “postage due” letter: I tried to give the carrier that amount due in stamps, and he wouldn’t take them. So I feel confident that the answer is no.

AstroChuck is our postal authority, though. Let’s see what he says.

It’s sort of an Apples & Oranges thing, and Seelix has a great answer. Even though we hand money over the counter and get stamps back at the Post Office the two are not interchangeable there. You might not have noticed it but there was no sales tax on your bubble mailer purchase, the PO is one of the few merchants exempt from adding sales tax to your bill.

No.