If you are registered with GST/QST, they have already advised you of the rate of tax you must charge to your buyers. Check carefully all the information given to you at time of registration.
In summary, the following table may help you when selling taxable products within Canada (except for QC which you should already know as rates went up last month):
NL 15% HST
ON NB NL 13% HST
BC 12% HST
Rest of Canada 5% GST
The tax is payable on the total amount of the transaction, including shipping, packing and handling charges.
You should consult your tax department for more information.
The rates shown above represent what you MUST remit to the government. You are free to absorb the tax if you wish (to be competitive), but
the tax must be remitted, whether you charge it or not.
No GST/HST/QST is payable when the goods are shipped outside Canada.
Also, I most strongly suggest you clearly state in your listings that GST/HST will be charged for Canadian buyers.