Unfortunately, tax rates are more complicated than that.  The rate of tax charged (GST or HST) depends on 
both the province where you ship from and where you ship to and the type of service you use.
From the Canada Post website:
Tax rules relating to interprovincial (consumer) Lettermail™ and (consumer) 
Parcel services are as follows:
subject to GST only if 
mailed from Manitoba, Prince Edward Island, Saskatchewan, Alberta or the Territories 
subject to GST only if 
mailed from Ontario, British Columbia, Quebec, New Brunswick, Nova Scotia or Newfoundland and Labrador, and the total price per transaction (single or multiple items) is $5.00 or more and taken at the counter at the time of payment 
if the total is less than $5.00, subject to GST and QST if mailed from Quebec, and HST if mailed from Ontario, British Columbia, New Brunswick, Nova Scotia or Newfoundland and Labrador. 
Tax rules relating to interprovincial 
Priority™ Next A.M., Xpresspost™ and (commercial) Parcel services are as follows:
subject to HST if 
mailed to Ontario, British Columbia, New Brunswick, Nova Scotia or Newfoundland and Labrador regardless of total price per transaction 
subject to GST only if 
mailed to the other provinces and territories.