thanks! I'll wait for your reply.
I tried adding an extra TAX CLASS called "GST only" and then created a new tax rate for "GST only" and set the tax rate at 5% GST (for SK buying books and audios) but when I tested it I get GST and GST so I deleted it for now.
Currently I have the following set up:

ZONE DEFINITIONS
Canada GST Zone GST (except Eastern Canada: NS, NB, NL)
Eastern Canada - HST Zone NS, NB, NL
Saskatchewan PST Zone PST (only when shipped to Saskatchewan)

TAX CLASSES
GST on Shipping
GST/HST/PST Class

TAX RATES:
GST on Shipping Eastern Canada - HST Zone 13%
GST on Shipping Saskatchewan PST Zone 5%
GST on Shipping Canada GST Zone 5%
GST/HST/PST Class Eastern Canada - HST Zone 13%
GST/HST/PST Class Canada GST Zone 5%
GST/HST/PST Class Saskatchewan PST Zone 5%