If you haven't already, put in the box you will be mailing it in and take to post office, have them weigh it and tell them the zip codes where it will be shipped to. That's the best answer I can come up with....??? :0)
find out the weight of the item in question (or a flat rate box if they have one in the right size) and go online to USPS.com and you can usually get a good idea of the shipping costs.