a fairly complicated shipping need

The system is going to sell furniture, will offer carrier choices like UPS and Fedex. The problem is, some of the products have more than one package, EACH of them has different weight and dimensions.
We are not sure if the carrier API addon can send out multiple set of these information to the carrier system, and may be get a (combined) rate back or something.
To make the matter worse, some packages are over 150lb, which we think we can only ship using freight service. We are not really sure how the whole thing is supposed to work.
Is make/mod the UPS and Fedex addon to automate the whole process with the info provide (package weights, dimensions, shipping address) doable?