Last I knew immigration office is on the right as you cross the border. Park your car and walk back to it. You can get your paperwork there and also pay for it. Don't forget to turn it in when you leave mexico (before the due date).
Stuart,
There is no way you can compare your site to Facebook. Facebook has 1.2 billion users. If you scaled this site up to those numbers there is no way you could provide this service for free. It would cost millions of dollars to handle the traffic.
As for a "love hate relationship" with...