OK, now I understand. When you said dropbox, you meant in Quick Base terminology a drop down field to make a choice.
So the way that this is done is that in your rates table, you will need to have a Key field which is the concatenation of the unique values which define the rate. The low tech way to do this is to have a formula field called [Rate Key formula] to calculate that string, perhaps like,
List ("-",[Zone],[weight floor], [Volume floor])
and then on the form have a form rule that says when the record is saved, change the value of a text field called [rate Key] to the value of [Rate Key formula]
Then on the child side you create the a similar formula field as use that as the reference field in a relationship to the Rates Table to automatically pull in the correct rate.
