Label printing & postage via Endicia
Other shopping carts have this option. It can't take more than a couple lines of code to add this feature.
TrimYou / TrimYouSpray.com commented
Go to MyShippingPartner.com and get an account. It's a fantastic webapp. that works as middleware between 1ShoppingCart and Endicia. It does exactly what you want, works great, great interface, etc.
Racing Secrets / Quarter jr commented
Here are a list of shopping carts with Endicia support http://www.endicia.com/Developers/IntegratedPartners/