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.