Re: Large shipping - How to split up shipping?
The basic shipping module won't do what you want. However, that's not to say that some customization won't get it to do so. I modified a site for a client last year who needed the exact ability you're seeking (although he uses UPS).
He can specify the number of cartons and weight of each carton for a product, and also specify if a particular carton(s) in that set can be used for other items as well. The shopping cart class was modified to do the hard work of deciding which items can be added together from those settings, and comes up with an array of cartons/weights. Then the shipping module gets a separate quote for each from UPS and adds them together for the final shipping charge.
You can see an example of how that carton information is displayed to the customer from this product page
Neville
An assumption is what you arrive at when you get tired of thinking...