Sales program data synchronization

Question

I am working on devopling some ideas for a program that would help my father-in-law in his business and I'd like to solicit some ideas on how to best go about this.

Basically the short of it is that the salesperson would carry a laptop to an appointment and record the customers name, address etc. The salesperson would also do some measurements and enter those, the program does the calculations and then the salesperson can print an invoice on the spot and have the customers sign it.

My problem is that I'm not sure the best way to get this info back to the database. When the salesperson has their laptop at a customer's house, they aren't going to have any sort of remote access to the server so I can't enter the info directly to the DB. What I was thinking is to have the client (laptop) export the info into one (or multiple) XML files and then have the program on the server import the XML into the database. Is this the best way to go about it? To me, this presents a lot of potential problems. It would make it difficult to dynamically generate a customer number as two customers could end up with the same number. There would also be problems if the salesperson didn't syncronize their machine with the server. It would be beneficial though in that there would be an XML file saved with the information and the information would be in the database.

Anyone have any suggestions or comments?