Not an expert but I've sold several smaller things on eBay.
One thing that drives up the cost of shipping is "return receipt". You need to track your package or a bidder can say they never received it. To do this you must ship Priority Mail with verification which can get up to $5 real quick. Add some special packaging like padded mailers, throw in your gas to get to the post office and you get to $7.95 quickly.
I keep a small calculator near my computer and I Add, Multiply, and Divide as I bid.
THIS WORKS FOR ME...