"tariffs aren’t paid by the exporter. With some minor qualifications, basically they’re paid for by the buyers — that is, a tariff on Mexican goods would be a tax on U.S. consumers. America, not Mexico, would therefore end up paying for the wall. "
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Wrong again Krugman. If U.S. consumers can find substitutes at a lower price than what the taxed Mexican products cost they'll buy those substitutes.  Mexicans will be forced to reduce the price to compete and to that extent bear the tax burden.
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You didn't know this Krugman?
Gee, this is sophomore level microeconomics.