How do others like the invoicing process that's used when a session package is used. Is there a use for all those new invoices?

The one issue I have is that every time I close a client session, it creates a new invoice with a new invoice number. So I have a 15 session package. One invoice for the total $ amount of the package is created, but then 14 other invoices, all $0, all with new invoice numbers, stack up waiting to go out when I close each individual session. So the invoice screen shows 15 individual invoices for 1 purchase. I was thinking of just deleting them after they're created, but they told me if I delete the invoice, the session will show back up as available. It sounds like they use invoices to drive the backend to do the inventory availability, because I can't see a business for all those invoices.