@kellyr2 

That's not a stupid question and I'm not sure my way is the best way.  To zero out invoices, I change the amounts on all products/services to $0.  If you delete the invoice, it would put any inventory items back in stock which you don't want.  Then, I make a note (in the description field of each product/service or one of the message boxes) with the original invoice amounts.  You can take a screenshot and save it as an attachment if you prefer.  That way, if the customer ever comes back and can pay the invoice, you have a copy of the amounts.