Cumulates Price in Header PO same item

Hi,

I have a problem with a customer demand.

He wants to make a PO with different item refer to a purchase requisition.

But the item is on the same material.

So for example

Item 1 Material A 10 ST 100 euros

Item 2 Material A 20 ST 180 euros.

The price is based on scales, so on item the prices will take the scales but i want to cumulate all the item price

and the header will take the price for 30 ST For the example not 280 euros but 270 euros. And recalculate price and item...

Is it possible with standard, or do i have to make a developpment on this ?

Thanks for your answer.