Non Catalog item requisitions - How to raise goods by amount?

I am aware that the line item type when creating a non catalog item is not configurable, I have had this confirmed by Oracle, and therefore the options are

1. Goods billed by quantity
2. Services billed by amount
3. Services billed by quantity

How does everybody go about raising a requisition for goods billed by quantity? obviously its not an option in the system, so how are these being recorded?

We have a supplier providing us expensive goods, and therefore we pay a percentage of the total at different stages of the production, say 10% at contract stage, a further 20% by a set milestone and so forth. How would you recommend I do this within the limitations of the system?