NOT to pay a fine that is sent directly to you at your home outside the EU is probably a bad idea. If you don't pay, the rental company will eventually pay the fine plus the late fees and pass that on to you via your credit card company. There is also evidence around that a debt collection agency called EMO ("European Municipal Outsourcing") is following up these fines quite aggressively.
Note that it won't help for you to cancel your credit card when you return home. The date on which you signed your rental agreement and gave your credit card details to the rental company will predate your cancellation and the car contract will still be honoured by the credit card company who, one way or another, will extract this money from you.