Sellers should build the cost of doing business, such as transaction fees and postage into the asking price of their merchandise. It's not that difficult to estimate shipping on most regular-size items. Variations up or down in your actual shipping cost based on destinations should balance out over time. USPS offers flat-rate shipping, one box one price.

Start with the highest price that covers all the shipping and transaction fees and then offer a discount for cash payments or personal pick up.