I would allow an extra hour at EZE incase its really busy.
A year ago when i had to pay the fine, it was 300 peso's, and they would only accept cash.
So i went to the immigration counter (the checking staff will tell you where to go), got the form, went to the ATM and withdrew 300 pesos, then paid at Banco National at the Airport using the form from immigrations, get the receipt, then take that to the immigration counter again, who then stamp it, and you then present it when you leave at immigration control.