For Ebay, you can plug in the weight into your listing if you do "calculated by buyer's location", and the Ebay shipping calculator will figure the buyer's zone and total charge, and add the appropriate fee

For Chipboard or other transactions, you just have to guestimate and add a flat average (knowing you may lose a little on postage on some transactions), or make postage an add-on to be calculated once you know the buyer's shipping address. For my website sales, I just do a flat $3 shipping rate, knowing I lose money on some postage