For my website I use tiered pricing for buying mutiples of the same product. Ranging from two to three price levels.
EG; if you buy five or fewer it would cost $3.00 per item, if you buy 6 to 10 of the product it would cost $2.00 per item, if you buy 11 to 100 it would cost $1.00 per item.
Could you incorporate a multi level pricing/tiered pricing/ quantity pricing option in the item set up, so when invoicing, prices would be appropirate for the ammount ordered.