It is interesting.
Here in the US we dealt with the free trade issue (for the former non-Canadian British colonies on our continent) way back in the late 18th century.
It all started because the colonial charter of Maryland gave it full control over the Potomac, right up to the Virginia shore. This led to squabbling. Also, there was a hodge-podge of currency, taxes, tarrifs, etc.
The Union eliminated all of that. The only thing a state could do is collect fees to support inspections.
I love how I can travel coast-to-coast with no need for papers, fees, etc. One currency. Yes, states have some control over things, but for the most part there is uniformity. Of course, having a fairly uniform culture and common language helps.
And in recent times free trade has expanded to Canada and Mexico, if I am not mistaken. My car was built in Canada!
