invoice number / order number

so we only want one number. We want the order number to be the same as the invoice number... I made them the same in the database... however If a person pays with papal and then paypal never confirms.. then the numbers get off so someone will have an order number of 50134 and an invoice number 50132.. This is bad because the email uses "order" number and the invoice uses "invoice" number. Why can't there just be one number so that if a customer is looking at there invoice I can look them up in my orders...

Please advise...