Product shipping assignment

I use ups and usps to ship. All items from warehouse one ship ups. All items in warehouse two ship usps. How can I assign items to a specific carrier. Thanks in advance for your help.

At this time, shipping is done at the "cart" level. There is a shipping by product mod that allows you to set a rate manually, but none at this time to allow different live rate carriers. This should maybe be a feature request to add individual shipping methods.. or maybe it could be done with a mod