Hi there,
Don't get into these cards buddy! Take your cash and simply cash in any local bank in Canada. Believe me you will be much better off by this than Post Office's zero commission where commission is taken out via unfavourable exchange rates. Call the local branch to get the best deal. Royal Bank of Canada (RBC) always offer better rates than others. No commission to pay and no hidden charges!!
You will always get better rate there than buying CDN dollars here in UK.
Remember to claim your 7% GST on your purchases NOT consumed in Canada. Keep all your receipts and get the GST brochures to know what you can claim!!!
Proud to be a Canadian !!!