You can't really do it free (only purchasing ETFs is free on questrade for the most part), and thus it is only worth doing on larger sums of money lets say $10,000 or more.
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To add to this, it cost me $600+ to do it with $10k in 2020. Forex is a bitch
QTrade is another option. They charge $8.75 to buy and sell most ETFs. I used them before and it worked fairly well. But their app and site seem to be a bit buggy. When I bought an ETF with USD, it immediately shows a big loss— it seems to show the price in CAD after converting with a bad exchange rate. But I can confirm that I effectively got market rate minus the two $8.75 fees when I’ve done it in the past.
The whole process takes a while: a few days for the money to make it into the account, then a day or two for the shares to settle, then a few days to journal the shares between the USD and CAD accounts, then another few days to sell and transfer the money. (The first time I did this, I think I also had to wait 10 days). It’s also annoying that you can’t journal the shares from the app. (Normally I try to do this stuff on a PC, but that’s not always an option given it takes many steps spread out over a week or two)
I haven’t tried it with Questrade. What do they charge to sell? And is it any faster than my experience with QTrade?
Edit: I think QTrade charges a recurring fee in addition to the $8.75 trading commissions, but it’s waived if you keep $25k with them
Desjardin's DISNAT offer free trades and free journaling, so quite hard to beat for Norbert's gambit, but you have to call them for journaling. You'll also have some fees if you don't meet their minimum investment amount.
Otherwise, NBDB offer free trades and $10 journaling and I think you can ask for it over secure messaging (but they also have a minimum investment amount), while IBKR offer instant currency exchange using very interesting rates (spread around 0.1-0.4 pip + 0.2 bps exchange fee with a minimum of US$2), but you can't use it only for currency conversion (against TOS, they'll close your account quickly otherwise).
Otherwise, Wise.com may be a good option with their 0.5% fee, depending on how much and how often you exchange money.