Reloading SUBE cards
I just went through this, and I thought I'd share the knowledge.
I've a SUBE card from previous trips, you can use these to pay fares on subways and city buses in BA. There are a bazillion loading points in the city, but you can only use cash at those.
To reload using a credit/debit card:
1) Get the Mercado Pago app
2) Register a credit/debit card
3) Tap on "Cargar SUBE"
4) Follow the prompts
5) Once the transaction succeeds, you will be told to tap the card at an official validation point. The SUBE app tells you where they are, most banks seem to have them, as well as city government offices of which there are many around the city. NOTE: tapping at a subway station to open the turnstile won't work to validate the load.
There are sometimes promo offers where you get 25% off (ie, you load AR$100 on the card, but you get charged AR$75).
Note that you can also use the Mercado Pago app to pay at merchants that normally don't take credit cards, by scanning a QR code. I did this at a few kiosks [1] with zero problems.
Hope this helps!
[1]: the ratio of candy sale points to other stores has to be the highest in the world