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In my experience, UPS will phone you at the number given as being yours on the shipping documents, offering to clear the package through customs for you. You have the right to clear it yourself and would save a brokerage fee by doing so, but you won't want to do this if you will only be in Toronto for a couple of days. Let them clear it. Understand this service could cost you $50 to $100 or more atop the taxes charged. (It's money UPS keeps for processing your package through customs.)
You can mention when they call you that you want to prepay the taxes. They should be able to call you or email you with the details. Alternatively, you could make arrangements with the business where your package is going to pay them for you. Typically, the taxes are collected at the point where the package is delivered.
You shouldn't have to pay duty (duty isn't charged on computers in Canada) but you will pay Ontario HST, a harmonized provincial/federal sales tax of 13% at the time I write this, plus whatever brokerage fee UPS charges you for the privilege.
I researched but it appears that neither Ontario nor Canada offer any sort of visitor tax rebate, except for certain expenses made for accommodations and transportation for organized tours of foreigners, which would not apply in your case (even if you were on such a tour, this purchase falls outside it).
Safe travels and I hope you enjoy Canada!