I sent something in a minimum sized padded orange-brown envelope. I stamped it via an automatic kiosk. I categorized it as a large envelope for postage purposes, because I couldn’t believe that this 2 oz thing was a “package,” but I realized too late that the machine asked to make sure it wasn’t “rigid,” which I now think it is (the contents certainly aren’t easily, and shouldn’t be, bent).
Will it probably get cross country without getting mangled? Will extra postage be owed at some point?
My guess (as a long-time ebay seller) is: it will probably make it OK without getting mangled, and most likely without postage due.
That said, if it gets kicked out of the automated sorter, it might come postage due.
So do things get automatically sorted out or is this at the discretion of your post office? For example, a lot of fancy greeting cards say “extra postage may be required!” but I don’t think I’ve ever had something returned or arrive postage due for that reason.
I think it works more-or-less like this: if the item is not explicitly marked “non-machinable,” it will get sorted with the rest of the mail. If it makes it through the sorting machine - great! If it gets kicked out due to thickness or size, it will get hand-sorted, and may get a surcharge stamped on it. If it’s close, and passes into the machine, but then gets damaged, it will come in pieces in a bag, with the surcharge.