If your question is whether the extended price for an item can be automatically calculated based on the amount of previous items on an invoice, the answer is no. All items are handled independantly in regards to each other (with the exception of 'subtotal' type items).

For either Quik Invoice Plus (DOS) or Quik Invoice for Windows (or Adagio Invoices), the 'Total invoice amount before tax' will be displayed on the Totals screen of the invoice. You could use that for reference in calculating a commission for all items.

Should the commission be for only some items on the invoice, you do have to calculate the base amount manually. You can use the F5 function key to bring up a calculator, and have the result automatically entered into the field.