This might seem obvious, but it wasn't mentioned in the episode.
When purchasing an item that has a mail-in rebate, you are paying sales tax on the pre-rabate amount, and you don't get that tax back. I don't hold it against retailers, they're doing their job and have to report the sales tax they collect to the government, but I get steamed that on my net purchase price, I am paying a much higher sales tax, and it is one reason I carefully consider what items I will purchase with a rebate.
This might not seem like a big deal in the US, but in Canada, and Quebec in particular, the government double-dips on sales tax (they charge the GST first, then charge QST on top of the previously taxed amount, but I believe that Quebec is the only province so inclined to screw consumers). For example, if I see a product that is regularly $100 offered with a $75 mail-in rebate, I am paying $13.95 in tax on the $100, and not the net $25 cost. When looking at the final cost, I might only be paying $25, but I have to eat over 50% of the final cost in sales tax. If the product came with a $75 instant rebate, I would only pay $3.49 in sales tax.
One final point that will only interest Canadians: many rebates come from the US. We often see $75 rebates advertised, but the small print reads "$50 USD converted to $75 CDN". During the time that the rebate is processed, the exchange rate can shift, which affects the actual value of the rebate when you cash the check.