Creating a Purchase Order, is there any help out there?

I am creating a purchase order for our sales team that can be utilized on iOS. I have two issues:

1) Some of our sales will be exempt and will need to be toggled off. How do you go about doing this while not destroying the end total?
2) Our sales tax varies due to county's jurisdiction. Is there a way to change that once the form is ready to be filled out?

Thanks Guys

Below is how my form is being set up, mind you this is an import from excel.

Image may be NSFW.
Clik here to view.