Customer requires a quantity based blanket purchase agreement at header (commodity centric business)

Summary:

Looking for a solution to track BPAs where the deliverable is gross weight is spread (not-equally/not known) across multiple items. Need to be able to track the decrement of quantity/weight at the header level.

Customer buys commodities in bulk where their current agreements' deliverables are based on total delivery of quantity "weight," not dollar amount. Dollar amount is tracked, but it's not the measure of importance. For instance, a BPA could have multiple line items where each item has it's own unit price and they share the UOM; the total of the 4 items will make up a delivery of 10 million pounds over 1 year, for example. The Quantity and mix of ordered items cannot be predetermined as the items are interchangeable and, further, the gross weight can't be divided between the total