Seeing actual names of things makes it easier to keep the relationships in mind. I'm still a bit puzzled by the exact problem you're trying to solve, in the absence of some key details. For example, you mention that "second table tracks all the various volumes shipped, and the cost for sending those volumes." This would imply that the second table records actual shipments, not available prices for future shipments. In that case, I think this model is probably denormalized such that it won't represent everything.
However, as I said, I'm not sure I understand the whole thing. I'll bow out for now, in the absence of more detail. I'd hate to make things worse.