ASP "Control Stock Level" Calculation

Hai,

I am developing a simple ASP-Access system for invoicing purpose, and the system is in 98% progress. What I want to do now is to add some sort of "control stock levels".

Let say I have set the quantity of an item/product (eg, pencil=100) in my database (in my database it refer to the table tblProducts with the field called PStockLevel) When one customer want to buy 10 pencil, the system will automatically set the PStockLevel to 90. All the calculation will be done in the page called tasks.asp

Coulde anyone help me.
If somebody want to use the system, go ahead and use it.
Thanks.

invoice.zip