Promotion code in sales order

Hello,

while creating a Sales order, i would like to apply a promotion code to it.
I can attach a promotion code with the setPromoCode(id) operation and it works, but the discount provided by my promotion code does not reflect in the total of the sales order.

I am using Java webservices 2.6
Here is my code :

...
//set my list of items in the sales order
so.setItemList(salesOrderItemList);
//create the promo code object and apply it to the sales order
RecordRef promoId = new RecordRef();
promoId.setInternalId("150");
so.setPromoCode(promoId);
//add the sales order
WriteResponse writeRes = _port.add(so);
...

In my example, my item is worth $99.00 and my promocode is supposed to offer a discount of $99.00, so my total should be $0.00, but it still shows $99.00.