Ground Advantage prices are zoned; the price will vary based on the distance between your location and you buyer's. Therefore, if you want to charge an accurate price, you should set up your listing with calculated shipping so the listing will automatically show the correct postage cost from your ZIP Code to that of any member who looks at your listing. If you use the shipping calculator in your listing, you just enter the weight and dimensions, and eBay does the rest for you.
