@ArmyRick, there are no trade "tariffs" between provinces. You have trade barriers, which is quite different, and they usually arise out of the exercise of proper Provincial powers, but with side effects on the other provinces. It's a feature, not a bug, of our constitution, which is not to say that they should be reduced to the fullest extent possible.
For instance, you create barriers merely by adopting a standard in each province for something, and then, in each province, forcing the supplier to demonstrate compliance of your product before it can be sold there. This can lead to either higher prices for the product due to the numerous compliance requirements, or a decision altogether by a supplier form province "A" not to sell in province "B" because they don't want to have to "make" that province's standard and they can more easily sell their product "as is" to the US (often the case) or elsewhere.
This is an international case, but here is how it happens: Remember those days when there were dealership in Canada for Citroen, Renault and Peugeot? Where did they go? Well, Canada adopted much higher and more stringent standards for rust protection and resistance for automobiles and the French car manufacturer decided it was not worth their wile to meet them for the volume of sales in Canada, so they simply exited the market.
With every province establishing their own standard in various fields, that's how inter-provincial trade gets hampered. Another way, of course is when the commercial sector is actually in the hands of the provincial government, such as for Alcohol in just about every province, thus limiting direct "producer to retailer" contracting.
Since provinces, no matter what they say otherwise, are jealous of their powers and exercise them in numerous fields (just about every field actually, as we live in a nanny state), resolving this is near intractable. But even more than that, it is simply not a field where the Federal government has any power or capacity to act, except perhaps as the "organizer-mediator' of inter-provincial talks. So both Carney and Poilievre can talk the big talk on that, but neither can deliver. It's up to the provinces to resolve amongst themselves (and we know how that usually goes well

).