While not defending outrageous postage costs I will say that it can be hard to quote postage costs.
International is easy as it goes on weight only, you just have to allow for the packaging.
National however goes on size as well as weight which can depend on what mood the postage workers are in & their ability to use a tape measure. If you use the website it will often not allow the item to be posted because it is too big, but will be taken at the post office without problems. I often quote postage for products but find the actual cost is different to the website quote. Sometimes I win, sometimes I lose. Usually +- $10.00 or so.
You may find some of these sellers have been caught out before & don't understand how the system works, these guys will probably come around with some e-mails, the ones that don't are just greedy.
I sold a set of universal footpegs on ebay a few years ago, I listed them at $1.00, sat back & waited for the bidding war that was to start my early retirement. After a week, in the last minute somebody bid $1.00 for them
I stitched him up on the postage though, I charged him $4.50 but when I got to the post office I found it only cost me $4.40