Ok so basically, i am using excel for stock purposes.
I insert the customers information, what product is being bought etc.. then
i want it to take away from the stock i have.
For example

Say i am selling Microsoft excel 2007 (Ironic isn't it?)
I would need it to recognise the products used in shipping.. (a4 envolope,
1st class stamp)

Say i have the column 'A', if i enter Microsoft Excel 2007 into there (From
a dropdown list)
and then the envolope quantity in 'B5' and stamps 'B6'

Is there any way how i can take away the envolope quantity and stamps
quantity by 1 when i enter 'Microsoft Excel 2007' into the column A

Thanks for your help