RemoveItem - exchange

Hi, I have a question. I want to add a function that removes items from the trade using the right mouse button, and everything works fine when there are 1-slot items in the trade. However, when I add 2-slot items to the first 4 slots and 4 one-slot items, I can't remove the ones at the bottom. I added logs, and it turns out that I'm trying to remove from slot 11. My grid is 4x3, and the last item is then at 7. Do you know how to modify the RemoveItem function in Exchange to take into account the size of the items in the trade grid?