Be careful with using BMO debit cards, TD VISA or Scotiabank Amex card. (other banks may also use this policy but I am not sure about it - I am sure about BMO, TD and Scotiabank).
BMO debit card and Scotiabank Amex does this - if you charge an amount in a foreign currency (that is NOT the US dollar, eg. the Euro), the bank will first convert the funds to US$'s AND THEN to Cdn $'s). Therefore, there are two conversions and therefore it's more expensive to the customer. This is in addition to the 2.5% handling fee.
TD VISA designates 5 world currencies (ie. US$, the Euro etc). ALL OTHER currencies are first converted to US$'s and then to the Cdn$. Again, the 2.5% fee applies in addition.