Well, I've talked to the postmaster general in my area (she's friends with my mom) and she said that pretty much everything is up to each individuals discretion. I once mailed a Priority envelope with a Priority stamp at the post office... it came back to me the next day due to lack of postage. The stamp was completely missing, but you could easily see the outline of where the stamp had been. My poor envelope hadn't made it out of my local post office before the stamp was gone!

I've never had any high amounts of postage due, but my carrier does make me pay for every penny. Literally. Once I paid a penny and once I paid 13 cents.

Oh, and the weight machine for the self-mail is completely different that the one the clerks use. The machine usually charges you more to cover possible "user error."

Yes, it is often cheaper to send an item in a far larger envelope than one it actually fits. The weight is distributed more evenly and the envelope may be easier to fit into a mail slot/box (or folded and shoved, whatever your carrier does!) Envelope size is generally a plus for customers unless your envelope is odd-shaped, extremly large or extremly small.

As for being too floppy? They were basically telling you that there was a strong possibility the envelope would be torn up by the machines, which in turn could possibly harm the machines and so you need to pay more in case this happens. That or you had an idiot (also a high possibility. Every post office has at least one.)

i think it's strange... sometimes i send matchboxes in envie under letter with object and sometimes they charge me $.60 cents sometimes $1 for the same thing and same weight...

recently my postal service is awesome anyone who send me packages with stamps..they don't mark them so I get to resend without adding postage b/c how can they tell. though no one has had a problem....but that's only a few times.

then my po moved. sometimes they are grouchy..but only when I send letters do they usually come back b/c they say the envie is weird shape....which it's not...weird really