Due to credit card companies rules, only Visa Credit and MasterCard Credit can be charged in USD for a company based in Canada.
All other cards —Amex, discover, Visa Debit, Mastercard Debit, prepaid card (Visa and MC), Diners Club, etc— can only be charged in Canadian currency.
The conversion rate on our end is determined by our bank, (currently Royalbank of Canada or RBC). Usually it's about 3% more than the money market exchange rate. We have no say in the conversion rate.
When your card gets charged in Canadian currency, if your card is not in CAD, then your issuing bank will convert the charge back to your currency for you. And banks being banks, your bank will charge a roughly 3% conversion fee back to USD. So the charge that ends up on your card is anywhere between 5% and 7% higher than what is the price of the membership listed when calculated against the exchange market rates.
We can't avoid this for you. If you don't want to pay the conversion fees, then please use either USD Visa Credit or USD MasterCard Credit.