There's already a method that decrements the units in stock for a product.
In the order completion page, the order is retrieved from the ID on the querystring. If the StatusID of the order is "1", then that means the order is "waiting for payment". Then, the order is marked "confirmed", and the order is updated, during which the UnitsInStock for the product should be decremented by 1.
P.S. There are actually a couple of forums dedicated to questions about TheBeerHouse:
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