Creditpoint System 

If a customer purchases a certain number of items, then he gains credit points which are stored in his user's account. If he purchases more items at a later time, he can redeem credit points against the purchase price. The customer can gather credit points with each purchase.

creditpoints { 
    pricefactor = 0.5
    10.type     = price 
    10.prod.1   = 0.02 
    10.prod.101 = 0.04 
    10.prod.501 = 0.06 
}