I believe Interactive Brokers has the answer for you: You can invest your CAN directly in USD and then transfer back for free once a month as needed.
so no need to wonder which currency you trade in :-)
Haven't done it myself but highly considering it ATM
It's not free to do the currency exchange but it is nearly free, lower than anywhere else. Main downside to Interactive Brokers is $10 USD minimum monthly fee. Also pretty sure there's a $10,000 minimum deposit to start an account. Although if you have the money to start the account, it might make sense and be cheaper than other options.
There is one option that is the ideal choice for exchanging CAD/USD for virtually nothing and that's Norbert's Gambit:
http://canadiancouchpotato.com/2013/12/03/norberts-gambit-the-complete-guide/Norbert's Gambit as described above using Questrade is ideal if you are only a Canadian citizen and not also a "US person" (includes all U.S. citizens). Being a U.S. citizen means it's not advisable to buy a Canadian ETF because of the way you'll get punished by the IRS, even if it's for a few days to do the Norbert's Gambit process. You can still do Norbert's Gambit if you're a U.S. citizen but you'll have to use an interlisted stock (like a major Canadian bank) because of their different tax treatment. The downside is that one of these stocks is a lot less stable than DLR so you have risk waiting for the several day waiting period under Questrade. BMO Investorline and RBC Investing both allow you to have "automatic journaling" where you can buy and sell the interlisted stocks right after one another which reduces your risk to basically nothing. However I believe both of them will charge you $10 for all trades (buying and selling) making $20 extra a month on top of whatever other fees they have if you plan to do this every month (if less frequent then it might make more sense).
There is unfortunately no easy cheap way to regularly exchange CAD/USD. You have to either bite the bullet of the investing service fees or the bullet of the 2.5% you're likely to pay as an exchange rate anywhere other than Interactive Brokers. It's a tradeoff and an annoying quandary. I'd love to hear if anyone else has a better solution I haven't heard of.
