You'll not go through Customs - your baggage will have been checked through to Paris.
But, you will go through passport control as you'll be entering the Schengen Travel Area when you arrive in Iceland. You'll then move into the international (Schengen) departure lounge to catch your Paris flight.
In Paris, at Charles de Gaulle airport, you'll not have a passport control, but will have to pass through Customs with your baggage.
Thanks so much for your help! It's nice to know what to expect.
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