There is a way in which you avoid paying the sales tax on stamps. Sales taxes are rounded up or down at 0.5 cents. If the sales tax on an item that you are buying is less than have a cent then you don't pay tax.
In the province of Ontario where sales tax is 13% you will not have to pay tax on items that are 3 cents or less. So if you make individual purchases of 3 cent stamps then you will not end up paying tax because the price of 3.39 cents gets rounded down to 3 cents.
While this might not be practical to do, if you are really interested in saving money 18 of these stamps will allow you to mail a letter within Canada. You will save 7 cents on the mailing cost of a standard postage rate letter. Yes, you can fit 18 stamps on a standard sized envelope. Attach 5 stamps on the top row and 4 x4 grid of stamps below.