Here is my shipping setup. Might be easier to see how I have structured it and do something similar? These prices do not include any additional charge for packing materials, etc. I take that out of my end. As you get into the higher tiers of insured postage, there's a larger gap between actual cost and what I charge here - for instance, I think I overcharged by a buck or two on an insured package awhile back. When that happens, I just issue a refund or have them pick out some extra value.