When listing an item and setting the shipping price, you can set a flat rate based on weight or you can do it based on zip code of the buyer. There should be a link to the USPS Website to get an estimate. Also, if you have a scale to weigh the item you're selling you can go directly to usps.com/calculate postage:
http://www.usps.com/tools/calculatep...lculatepostage and enter the weight and zipcodes To/From where package is being shipped. You can do domestic and international.