You could add a script at some point in the invoice process to adjust stock levels.
Im not familiar enough with the starter solution but it seems the most reasonable way to do this quickly
Im assuming the relationship between your product table and the invoice line already exists.
You can then use a script triggered off one of the fields (probably lineitem qty) to update the related products qty count
Set variable [$lineitemqty; invoicelineitem::qty]
Go to Related Record [producttable]
Set Field [producttable:::qty, producttable::qty - $lineitemqty]
Go to Layout[original]
Somethiing like that dont quote it as gospel at the moment
Im sure the gurus here may have a better way to script this but the concept is the same.
The main thing I don't like about this starter solution is that it can only track current inventory levels. It doesn't show you how inventory levels are fluctuating over time--and this is often very valuable information to have. There's an alternative approach you can use based on an "inventory log" that will track such changes. You can adapt your lineitems table to function as this log.
Search this forum for Inventory Log and you'll find several threads on this subject.