Other than bridge or tunnel tolls, with a possible exception of which I am not aware, CBP land borders do not charge fees. This is really a public relations and convenience decision more than an economic one. To be sure, when purchasing an airline ticket, the following fees are charged per person:

• $5.50 - U.S. Customs User Fee
• $7.00 - U.S. Immigration User Fee
• $5.00 - U.S. APHIS User Fee

At land borders, the same 'services' are performed, but the user is not charged. Land border in many cases present more challenges than airport crossings among them the inspection of vehicles which can time-consuming and therefore costly; that is not something that happens at airports. In addition, airport borders have the luxury of APIS so targets can be met planeside and efforts can be even better-focussed.

Back to convenience and public relations: First of all, convenience. A lot of stuff happens at land borders and collection of fees would be an added step; at the San Ysidro crossing, wait times are already routinely over one hour at many times of day-- it would add time to collect fees.

The fees above (totaling $17.50 for the addition-challenged) are collected by the airlines when tickets are purchased and most people don't bother looking at the breakdown of all the taxes. After all, the taxes are usually a fraction of the total cost of the ticket. Imagine if CBP made everyone pay $17.50 cash when crossing a land border-- there would be an uproar! Wait in line for over an hour and pay $17.50 per person--what a lousy government!

Imagine if a family of four traveling from Des Moines to Orlando (with a stop) had to cough up $20 cash at the TSA checkpoint each way to be groped and radiated; travelers would be outraged. Stack it onto the tickets and people barely notice; this is the exact same thing.

I hope that sheds some light on the issue.