a method of managing invoices for goods services provided by several contractors working at a remote well site involves entering invoice data into a mobile computer at the well site and transmitting the data over a wireless communication link to a home base computer . in some forms of the invention , the invoice data is validated by transducer - generated data , which is produced by a transducer that senses a service operation being performed at the well site . passwords prevent unauthorized use . some information displayed on the computer at the well site reassures the contractors that the computers are communicating . the method is particularly useful when the contactors are independent contractors using different service vehicles in performing significantly different service operations .