In Ye Olden Dayes, the US Postal Service did not have minimum size requirements, and (I wish I had it now) I made a tightly folded letter that ended up in a triangular shape. Each space was tightly covered with drawings and writing, as was the "envelope" (the outer side of the final folded piece). I sent it to a friend who assumed I was round the bend. It was colorful and fun.

The USPS also used to deliver mail without postage, charging the recipient for it. Many of us had fun neglecting to put stamps on our bill payment envelopes.

History shows that the USPS did not like all that 70s nonsense. They changed the rules on us!
 

Once SJ sent me a plastic net bag (like the thing onions might come in) that had a variety of things in it including a rifle shell casing. There was a twisted wire stem of a paper leaf holding it shut and the stamps were stuck onto the leaf.

Try sending THAT today.

I was an avid letter-writer, and sent all kinds of decorated envelopes although I don't think anything too oddly shaped.