This transaction posted five days later at 3.46 CAD, a slightly higher amount than when it was pending. No refund, no nada.
Refund came today. Here's me trying to figure out the math for myself, I could be wrong
420 ARS purchase pended for 5 or 6 days at 3.38 CAD, made late night of Dec 3, Saturday/Sunday
Posted at 3.46 on December 9
2.5% Mastercard FX fee = 0.08
Refunded 1.52 on December 11
Cost for 420 ARS without FX fee = 1.86 CAD
Cost for 420 ARS with FX fee = 1.94 CAD
Blue CAD cost (Monday, Dec 5 rate), according to cuex.com = 1.83 CAD
ARS to CAD rate without FX fee > 1 ARS = .0044 CAD
ARS to CAD rate with FX fee > 1 ARS = .0046 CAD
Blue CAD rate on Dec 5, according to cuex.com > 1 ARS = .0043 CAD
If I were to make a 20,000 ARS purchase with my credit card on that date, it would have set me back $92 CAD on my card. In cash, it would have cost me around $86, without the WU transfer costs built in.
Say I wanted to make the same purchase today instead, the seller hadn't yet jacked up the price, and I made a 2-4 hour WU transfer from my Scotiabank account because I needed the thing right now: I would get a .0041 rate, plus a $5.82 CAD transfer fee, setting me back a total of $87.82 CAD. Of course, the exchange rate would be better today than 8 days back, and if you make bigger WU transfers it kind of amortizes (?) the transfer fee, but in the end, it all comes down to $4-5 CAD difference for an emergency purchase without thinking too much, or finding a decent WU that's open.
TLDR: Selon moi, for occasional purchases below $100 CAD, using MC is totally alright in a pinch.