I work on a module for add shipping price of a purchase order. My moduel works: the shipping is added on the purchase order and the invoice. But when I try to pay the invoice, the shipping just disappeared.

I saw the collumn balance (residual amount) who is under the amount.total of the invoice. It also, doesn't add the shipping price.

What is residual amount? Did I have to overide it for add my shipping price?

PS: I really need, an ansswers