Problem with custom Button on Tag along item

I need help on custom button. I have created custom button for Money Order and This item is a Tag Along Item because any time We sell money order We charge fee. The problem is When Cashier click on custom button for Money order and The blue Table box Where We scan the item is moving to 11nd line Where We have Tag along item. But It should move to 3rd empty line for next item. If my cashier do not move the box down using arrow down button on key board The new scan item will take over my tag along item How I fix this Problem