Like many people I overstayed my 90 day visa as I was happy to pay the 4500 peso. However when I the border into Brasil at Iguazú I was told I couldn't pay at the border, I needed to pay online. I was given a document, sent on my way to Brasil and advise to pay online.
Once I logged in it said I needed to pay 15000 peso as I left the country without paying my fee. When in fact I was still in the country when they gave me the documents regarding my fine.