Add row to scrolling window with Visual Studio Tools?

Hello All. I have the following business requirement: If a customer orders too few of an item, they are to be charged an additional fee. The additional fee may apply to more than one line item in SOP. I would like to evaluate the quantity ordered using VST. If the quantity is too low, I would like to add a new row to the scrolling SOP window. After about a day of research and trial and error, I am nowhere. Is there any way to navigate to the last row or a new row in a scrolling window using .net/VST? Thanks.