I realize that this is an old question, but it did appear as a top question on Google, so I thought others might be looking for this topic as well.

Route Trakker Route Sales Solution is a handheld pocket PC application that can track inventory, create invoices, help plan your sales routes and generate all kinds of customer reports.

It can also print invoices to bluetooth printers or send the invoice wirelessly via fax or modem. Plus there is a server component that runs at your office that can give real time data to the office about what is going on in the field.

You can find out more at Route Trakker Route Sales Software