If you are patient enough, I believe that you can also buy tickets with a credit card over the phone during office hours. Not sure if they will have you on hold for a long time.
I went with Direct Ferries. Got tired of faffing around trying to get either page to work. I haven’t used my ticket yet as I’m currently in Europe but will be using it in the coming weeks
The price may very well fluctuate in pesos but if you use a credit card in euros or dollars the price will remain the same. I am sure that you may buy the tickets in person the day before at any of the points of sale in Buenos Aires.