In a couple of previous post I did a walk through of the summary update options for invoices. There is an option to have automatic summary if there are different ways you want to do the summary update for different customers. You can turn on the summary update option in the accounts receivable parameters and then choose the fields relative to the document Quote, confirmation, Packing list, Packing slip, Invoice. Then on the customer master record you can choose if you want those automatic summary options to apply. This is a quick walk through an example with the sales confirmation and invoice.

Cheers

Lachlan