Get a Sapphire Reserve, you get 5x rewards points on travel purchases & 0% commissions (which most purchases in Argentina qualify as). The annual fee of 450 us pays for itself fairly quickly with all the travel perks.
That means with current visa rate, after you pay it off the real rate is 433.59 (412.94+5%). 433.59 is still 4.7% less than the 455 a cueva would pay, but to change money at a cueva, unless you live there, you also need to pay transport (taxi, gas, etc) & unless you go there everyday you're going to loose significant buying power by holding pesos. So 455 isn't the real rate & you save big $$$ only changing what you spend.
FYI "travel purchases" means plane tickets, hotels and similar, not everything as long as it's overseas.