Typically you would do this in the query. It depends on how you have designed everything and the business rules, but assuming you have a current output of:
(person, salestype,totalsalesamount) then add a step that joins from salestype to the commission table where you can pick up the commission % (say) which then can be used in a calculated field COMMAMT: [totalsalesamount * commrate / 100]. This is calculated for each record and can then be displayed how you like on your form/report.