On a sales order, how can we get product committed to 1 lot only?

We are in the chemical business and receive goods by location & lot.  One batch lot can be received into multiple locations.  Many of our customers demand we ship from "1 lot only".  This saves them the time/cost of having each lot analyzed on their end.

This being said, to ensure the customer only receives 1 lot, we manually hard commit the goods at order entry time (instead of having the pick slip hard commit and possibly picking different lots). 

The problem with this set up is the fact that our pricing is often based on FROM Quantities i.e. (from 10,000kg). (managed through advanced pricing as a line level adjustment/ also based on Order Detail groups to pull in various sales detail info like branch, pricing terms as price can differ based on these values)