How did the post office accept it like that? Flat rate boxes are supposed to at least close on their own without tape and be folded correctly. I've had one that was refused at the counter because I tape the flap of the flat rate box down. Small flat rate boxes can fit inside a flat rate envelope so it wasn't a problem
And a friend of mine puts the label over the flap of the flat rate envelope and had one returned.
So them accepting one where the stuff inside doesn't actually fit is strange.
The attitude that shipping is supposed to cover his selling expenses is just wrong. If he'd justified higher shipping because of using a bigger box or claiming the expense of packing materials plus some handling. I might buy that, but not the way he shipped it.
I'm not sure what I'd do for feedback. I've basically abandoned feedback except for sellers who ask or go well beyond normal expectations. Or sometimes for a newish seller who asks.
Steve b