What I do is send everything Priority, you get the supplies free from the PO and you can order them online. Most items are less than a pound and fit in the bags or boxes they give you. You do it right from paypal if its an ebay auction, if not you can go to
www.usps.com and do it as well. You can get a cheap scale from Office Depot too. Its so easy, I never go to the PO unless its international.
The thing I love about paypal is you can print it on plain white paper ande then all you have to do is cut it and tape it to the package, so easy and with priority you get free Delivery Confirmation.