Do you have a scale at home for postage? I have a smaller scale I bought from Walmart at about $20.00. It's good for up to 5 lbs I guess.
I add all my handling fees, e,g, cost of envelope, ink for printer, paper for printer and I know a little about taxes so I have a new computer so I divide the cost of the computer by 24 months. I use it a lot. Besides it will be obselete in the next 2 years. I add that in and gas/ mileage to the post office and back I will guess at some misc cost like elec, heat, water yu name it. I just add an extra Makes your head spin, don't it. Well here's the shorter version. Add all supplies add extra 1.50 for misc(elec, gas, water, ink paper, computer, and so forth) that's my shipping cost. Also don't forget he insurance if you're sending something over ten dollars. I insure all my international packages and some form right here, depending on the cost to replace.