When I sell on eBay I list the shipping weight in the text. The customer is free to calculate what he thinks it should cost. Sometimes the results are surprising. The weight of most stuff isn't hard to find or estimate. For large or heavy shipments, boxes and supplies can be a significant cost if purchased retail. A decent sized box can run 3 or 4 bucks easily.
Some sellers use flat-rate USPS whether that is the lowest cost or not. It often is not.
If you think the calculated shipping is too high, ask BEFORE you bid.
The weight is only ONE factor in figuring shipping cost, I recently sold some Passenger cars, to 3 different buyers on this F/ST board, and all three shipping prices were GREATER than the shipping rate for the weight. The shipping companies use a weight/cubic foot scale as well as simple weight to figure shipping rates. On one of my sales, the actual weight was 15#, the DIMENSIONAL WEIGHT charge was 31#, for the package.
It had been too long since I had sold/shipped anything, and all the shipping rates were about double what I expected, I wont guesstimate shipping in the future, but I did honor the shipping prices that I quoted my buyers.
I can understand charging a LITTLE over actual postage, to cover boxes, bubble wrap, peanuts, tape etc., but I have seen sellers that use shipping charges to pad the profit margin on some items, this was especially common with ebay before they started charging final value fees on shipping charges. Some sellers used to sell items way below the true value of the item, then charge ridiculous shipping fees to make it up. Sell a $100 item that would cost $8 to ship for $8 dollars and then charge $100 for shipping, the end cost to the buyer is the same, but the ebay fees were only based on the $8 "SALE" price. Needless to say, ebay eventually caught on to that trick.
Doug