Math Operations On Values Question

I have created a table that holds the inventory information of some items i have in stock. What I want to do is this:

1.)When a customer orders something, i enter the quantity ordered and hit a process button. When this button is clicked it automatically reduces my inventory count to reflect the reduction

2.)When I receive shipments, i want to enter the information and hit process so that it automatically sets my inventory counts to reflect the addition of the new units

I am having trouble figuring out how to go about doing this. I was thinking of somehow setting up some textfields and then adding buttons that set fields accordingly.

Any help would be most appreciated.