So Mexicans start using bitcoin, amazon gift cards, or something like that. Or we end up with "underground banks" that can effectively transfer money across the border without actually doing so, i.e., you buy a digital good at a physical location in the US, transfer it to a relative in Mexico, who then sells it back to the company at a slight discount for cash (which, as a side-effect, would be a great way to launder drug money).

Or I guess, you bounce money through Belize or something.

Either way, it doesn't sound very effective.