Yes, but in the past taking advantage of these significantly cheaper prices (say 20% or 30% cheaper) also took significant effort. Enough so that it was (and still is) only common on big ticket items (like a car) where 20 or 30 percent translated to a lot of money. And, although common with these big ticket items, not common enough to impact the economy. ie. Car dealers are not closing shop due to cheaper cars in the US.
Now, however, it's all too convenient to buy, for example, that iPhone case from the US instead of Canada because shipping from the US is (no, not 20 or 30 % cheaper) half the price!!! This is so much an issue that it IS impacting Canadian e-commerce. Online retailers are commonly, [A] closing shop because high shipping costs don't let them compete with US retailers, or driving to the US to ship products TO Canada. There are even companies starting in Canada just to target this issue, claiming that they'll ship what you give them using USPS (basically just drive your package across the border for you).
E-commerce is a growing business in Canada. Unless we want to see it all move elsewhere (like manufacturing and many other things already have), I'm sure Canada Post can do at least a little better than double the price of their neighbouring competitor. People are willing to pay a little more for faster (domestic) shipping, but double the price is not a little, nor 20%, no 30% more.