Many Canadian on-line sellers will not collect PROVINCIAL Sales Tax on shipments going outside their own province. They should still be collecting the GST at 5%, however. Technically YOU are supposed to be an honourable citizen of Ontario and take your invoices to the nearest Ontario government office and pay them the extra Ontario Sales Tax, but you can guess how many people actually do that. In fact, I'm not sure whether you can actually do that now that Ontario has no Provincial Sales Tax, and only a HST administered by the federal GST people.
I live in Ontario, and often buy from on-line USA sellers and have goods shipped to a package handling company just south of the border - it's a 40 minute one-way trip from my home to them. They will hold a package for a few weeks at least, and charge US$3 per package when you pick it up. This means you MUST have a Canadian passport with you to enter the USA. When you return to Canada you must stop at Canada Customs and declare how long you've been out of the country and what you are bringing back. Assuming you are honest about this, they are supposed to charge you the appropriate duty and HST on those goods. Usually I find the duty rate on computer parts is nothing and you pay HST. But it seems there is some minimum amount (maybe $100 but I can't figure it out) below which they just don't care and tell you to go away. So SOMETIMES you avoid paying the tax that way. Even if you do pay the HST, though, sometimes the price is better, and sometimes that's the best way to get exactly the parts you want.
Here's another small point about the process I do. It costs me my time and some gas to make the round trip to the border to pick stuff up, so I try to set up several items for pickup at the same time. I don't worry about my time. But before I make the trip I arrange to have my car's gas tank low so there's just enough in it to get me there. After picking up my packages I fill up at the US gas bar next door, and their gas prices are much less. In my case, it works out that the money I save on a tank of gas this way pretty much covers the gas I spend to make the trip, so I come out even.
Oh, another important little hint. The package service company tells me (and all their customers) to tell the SENDER of the package that my address is their address. That is, to the sender, I live in the USA near the border at the package company's address. But that can be a wrinkle for paying by credit card, because some on-line firms' systems will NOT ship to an address different from that on the credit card - to prevent credit card fraud. So I have arranged with my credit card company to place that USA street address (where the package company is) on my credit card file as an Alternate Address. This seems to let the automated credit card processing systems accept that address as a legitimate place to ship to.