E1 Rounding qty sales roder from EA to Box

Hi group

E1 9.2.

Customer always order in one unit of measure. Let's say EA.

Item are packed in box, and the content depends on the item.

For each item conversion is fixed.

Item A : 1 BX = 10 EA

Item B : 1 BX = 15 EA

Item C : 1 BX = 50 EA

and so on ...

But Customer do know the conversion by item. He just order in EA.

When shipping, all the BX have to be full.

Example : Customer order 35 EA of item A. This need 4 BX. So the shipping qty will be 40.

What is the best way to manage this in E1 ?

- Change the qty in the order and put 40 after agreement with the customer ?