Thank you for your help and I appreciate your input.
I am using express checkout so the transaction or payment is always processed on my website and the transactions always have a unique ID.
This is exactly what I want to do, however I can’t figure out how to add the update behaviour that subtracts the amount ordered from the Stock Colum
($stock = ($stock - $purchased_amount))
Is this something I do in the eCart Object Calculations?
Thanks in advance.