eBay don't take any of the postage cost? how can they? unless you're using their flat rate post maybe. Ive only ever shipped parcels that are a measured weight and dimension.
I rarely have issue with the estimated shipping being incorrect. I just find it a drag when I'm using multiple different boxes to go in and edit the box size and weight every time I list a new item.
Last week I had someone offer me $11 each for two items I've got listed at $28 and $35 respectively. they might be a little high but they are certainly worth a lot more than $11 each. the offer didn't even ask for a combined postage quote, which I personally would have asked for before making a ridiculous pair of offers.