I don't sell things on Amazon, but what I find odd is that when I buy books from UK Amazon sellers, the postage costs are the same for tiny books as for huge heavy books, and also the same for books posted from the UK as they are for books posted from the States. And if I buy two items from the same seller, which come in the same package, I still have to pay two lots of postage. So it makes no sense.
I've bought huge heavy textbooks from American sellers, and I'm sure their postage cost way more than the standard £2.75 (but then I imagine they still must be making more money than they'd make from selling on the US Amazon, because otherwise why would they be selling on the UK one?). And equally I've bought little books from the UK, where the postage would have cost way less than £2.75. It seems a daft way of doing it.