If you are shipping USA to USA and running Zen Cart 1.5.1 or earlier, you can use Dropship Core (this includes support not only for specifying different origins for shipments - but also shipping dimensions). It comes with a basic example "UPS Parcel" shipping module.

I am working on a newer version of DropShip Core (international support, more flexibility, less intrusive, 1.5.2-1.5.4 support, etc). Currently this project is a low priority (as my time is limited and the project has no sponsor).