Trade barriers

American free-traders like using the reductio of some state, Alabama, let's say, erecting trade barriers against Mississippi. The Commerce Clause prevents such shenanigans, but the obvious ridiculousness of having trade walls within a country helps illustrate the case against having them at the international level.

Unless you're in Canada where interprovincial trade barriers are the norm. Here's a nice bit of provincialism:

He’s [Tim Hudak, Ontario’s Progressive Conservative would-be premier]  promising to kill a $122-million contract to refurbish Greater Toronto Area transit agency GO Transit’s buses because it went to a Quebec firm. That company offered the low bid, but alas, it would cost jobs in North Bay. “Losing 109 jobs in North Bay is like losing 1,000 jobs in Ottawa,” local PC candidate Vic Fedeli somewhat obliquely told the Toronto Star.

New Zealand has no trade barriers between its islands; the main trade barrier with the rest of the world is distance.