Well, Leche, don't forget WHO is fining you. The rental company does not fine if you don't have an int'l driver's license... it's the Civil Guard or the police.
I'll tell you a story. A few days ago, my car's insurance policy expired. Two days before I phone the insurance company and they told me not to worry. They say they allow up to one month to pay the bill and I would have insurance even during the period of time while my insurance was not paid.
However, I wasn't happy with the answer I got and I went to the insurance company to pay the bill the same day it expired. I asked them: "Your colleague on the phone told me I had insurance even if it took me one month to pay. And what if the police stops me and I haven't paid the bill yet?. I could only show them my old receipt for the previous year!!. Well, in that case... you'll get fined" Oh, thank you for not telling me about it, I thought.
SO -> you should always check what the rules / laws are