Purchase Order Amount Override Invoice Amount

We are a service industry and do not have any inventory.  However, we do have orders that we track.  When we receive vendor invoices they often do not match the order.  More often than not, our bill created from our internal numbers is more accurate than the invoice we receive from vendors, so we use it to create our payable.  The problem is that when our vendors have questions, we do not have the invoice information within NetSuite to easily look up (we currently track this in excel).  We are not currently using the PO module because as far as