There should be a way to recalculate cost of items received after dicount percent. example: say you pay $10.00 for a particular part and you get 5% off for early payment, the cost is now $9.50 and should be updated in the cost section automatically. If you have several items ordered (50 for example) and you receive a $25.00 discount on that order, that discount should should divide itself by the number of items ordered and reflect the new cost. in this case $0.50 off per item.
Just a thought