condition type in purchase order

Hi experts !

In a purchase order (ME21N), i use a Freight condition type with fixed amount. I explain with an example :

purchase order with one item : quantity=100 pieces - price=1u20AC/piece - fixed amount of freight=100u20AC

1/ standard SAP behavior

Good receipt n° 1...30 pieces....so in the accountable document...freight=30u20AC

Good receipt n° 2...70 pieces....so in the accountable document...freight=70u20AC

2/ The customer need

for each good receipt, SAP have to invoice the fixed amount of freight

Good receipt n° 1...30 pieces....so in the accountable document...freight=100u20AC

Good receipt n° 2...70 pieces....so in the accountable document...freight=100u20AC

How do they do it ?

Thanks a lot for your help

jte