Does sound intriguing. I once provided guidance to someone implementing UPS Hundred Weight Service which involved some similar logic. Basically we had to come up with a way to load up pallets based on what was in the cart and if there was room on another pallet determine which of the remaining products would fit on the pallet without putting it over weight and do so while minimizing the number of pallets that would need to be used overall.
That one was a bit complex, but in your case I think you would just need to implement some logic in the RTShipping class to look at the items in the cart and then determine what to do with them. You could probably get away with only touching that one file.
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