I don't sell a lot of stuff, but have sold a few items via ebay and the forum. Prior to listing the item, I find a box, pack it up with adequate padding (don't seal it), take it to the machine at the post office and enter the farthest CONUS zip code to get a price that should be near the maximum. Then I based the shipping cost on that number and included it in the listing. I might make or lose a couple of bucks on the shipping - not a big deal.
This method isn't viable for someone who sells a lot of stuff or lives a long way from a post office.