Well, what I do is package up the item you're shipping and then weigh it and estimate shipping to the farthest part of the country using usps.com - I live in Florida so I use a zipcode from the west coast like California, so I know the most it will cost to ship, then if you're adding delivery confirm, etc tack that on and it'll give you a good idea of what to accurately charge. I sent a 12.oz kinda heavy/bulky thing out today with confirmation for $3.21 so I'm sure you could get it cheaper without out of pocket - you'll get higher bids that way :-)
Don't short yourself if it costs more, but just saying it'll help your bid since people like cheap or free on here, lol. I fanned you, if you need any help let me know! I'll be watching this and hopefully bidding later :)