Fuel for shipping may be on an upward cost trend but shipping is more energy efficient than more or less any method of transportation requiring an engine. Trucking stuff across the border takes a lot more fuel per unit weight.
I doubt the point on immigrants too; Mexican enforcement of immigration laws is a great deal more vigorous than its US equivalents.
There's almost no foreign transport costs for manufacturing just south of the US/Mexico border. Fuel for shipping can only go up over time.
If Mexican workers get too expensive, Mexico would find it easier politically to take in immigrants from poorer countries than the US would.
Then there's Turkey for Europe.