One of the main issues with the margin business model (Profit of 5% of a payment for example) is that fraud is leveraged. This means that when you lose 100% of a transaction due to a chargeback or fraud loss, it takes you 20 non-fraud loss transactions to make up for it. The fraud leverage is a huge issue for platforms like this, and in certain countries half the transactions can be fraudsters.
This is silly. US is Canada's biggest trading partner and one of the largest borders in the world. 80% of people live within 1 hour of the border. Free trade has allowed both economies to prosper. Canada has everything to gain with free trade. You are right on over reliance, but free trade benefits everyone.
When you spend money on manufacturing and equipment, a company does not get to write off the full amount against their taxes when they buy it - they may have to spread it over a period of useful life of that equipment.