I downloaded this nw shipping assistant, and it is really cool. You can print shipping labels, then just take them to the PO and pay for it. You need a scale to weigh packages, you can use you own, you dont need a PO one, as long as its correct. By usign this feature you get Delivery Conformation added for free.
http://www.usps.com/shippingassistant/welcome.htm
http://www.usps.com/shippingassistant/welcome.htm