Mary, the first thing you need to do is buy an inexpensive scale on Ebay..you're not allowed to up your shipping price if you make a mistake. I found a great link in Ebay where you can figure out weightand prices:
http://payments.ebay.com/ws/eBayISAPI.dll?EmitSellerShippingCalculator
Remember one thing..if it's a small pkg, 13 oz or under, you can ship first class and it's not expensive at all. You can also just drop it in the mailbox. If it's a pound or over, it will have to go to the PO. I think you better do some reading here:
http://cgi6.ebay.com/ws/eBayISAPI.dll?MfcISAPICommand=S...searchString=postage
and here
http://cgi6.ebay.com/ws/eBayISAPI.dll?MfcISAPICommand=S...earchString=shipping