Like I said, it's complex - for our internal solution we just queried for rates for all of our box sizes, and then our software suggests the cheapest it thinks

Like I said, it's complex - for our internal solution we just queried for rates for all of our box sizes, and then our software suggests the cheapest it thinks will fit, but allows the warehouse guy to override it. It has a really simple formula for "learning" when he chooses something different. A general solution would be much harder, but if it had existed we would have forked over money without too much thought.