Sales Tax - How to

We sell capital equipment to companies and individuals. We are required to collect sales tax unless the entity to which we sell is tax exempt. However, sometimes we don't know that the entity is tax exempt when we generate the invoice. If we create the invoice in one accounting period and then find out the customer is exempt in the next, the practice we've been taking is to credit the entire invoice, and then create a new invoice showing the sale minus tax.

While this method works, I can't imagine that it's correct from a GAAP perspective. Does Netsuite have another method that will allow us to track that sales tax and the credit and have it flow back to the accounting period to which it comes from? Or, another question is how can we handle this without the necessity to credit the order and re-invoice it?