Canadian residents pay duty on imports from the USA valued over $150 and taxes over $40.
So assume you will pay at least 5% GST on any purchase.
That's the short version.
There are different standards on imports from different countries. If you import the same hardware from Germany, you would be paying both duty and sales tax on anything valued over $20. Seriously. Twenty.
Then there is 'customs brokerage'. You are best off if your seller ships by USPS/Canada Post because the brokerage/service fee will be only $9.95.
Most couriers are even higher, starting at about $25. And they will not hand over the goods until that is paid.
You should also consider sellers who use the Global Shipping Program which is much despised here.
Most of the hatred came before the new NAFTA agreement raised our duty and tax -free allowances. Canadians for complex and now defunct reasons, were astounded to learn that there actually were import fees on US imports and infuriated that they were being charged.
The GSP is a Seller Protection program, in the form of a freight forwarder. It charges import fees* before the seller is even told to ship.
This means that when your item arrives, there are no added fees (because they are already paid). You will be told what those fees are before your complete your payment. Pay attention!
*If you are buying for a business, the GSP will not break down those fees. They specifically do not do B2B transactions, whatever you and your seller may think.
