On the other side of the ledger...
A seller in the US had 4 items l wanted to buy, total cost of the items was $110 AU
If the items were posted separately as per the postage quote the cost would have been $206 AU
l contacted the seller and asked them for a postage quote for the items (bash plate, 2 blinkers and a rubber muffler mount) and the came back with $75 total, if its more then they would wear it and if its less they would refund the difference.
The seller sent me an invoice for the 4 items and postage of $75 and paid that through PayPal as usual. This is how any change to price or postage is done that I've found, they just send an invoice the reflects what's been negotiated.
l got a refund on the postage of $13
Another seller l bought some decals from wanted $26 for postage. l contacted them and told them there postage cost must be wrong and got the postage for $9.
l don't know if the seller is compelled to use the ebay postage system but l always ask for a quote if l think the cost is excessive.
Sometimes it works and sometimes l have to suck it up !