Hi David,
If you edit the stock Quickbooks_Estimate screen (used in Method for Quickbooks Desktop) you should find the Create Sales Order action set within the More actions... button. It might help to reverse engineer this action set. There is a Basic Math Calculation action within this action set that calculates the sales order rate (sales price). You can use a similar action to calculate the rate for your invoice. There are also actions called Insert Records into Table which are used to insert data from the active estimate to the new sales order, or in your case to the new invoice.
Below is a screenshot of the Basic Math Calculation action within the Create Sales Order action set. This action result can later be inserted into the invoice table as the rate. Hope this helps!
Jason