A.P.C. sells the best clothes (I would wear anything there!) and also has the calm and minimalist atmosphere that you would love. Their original store and warehouse along Rue Madame, near Jardin du Luxembourg, are definitely work checking out. I regret not taking more pictures!
Got myself a bottle of Fleur d'Oranger as a souvenir (for 20E only! I was shocked), boyfriend says it's the best fragrance I've ever owned. I love how it evokes an air of orange blossoms with a hint of jasmine flower.
38 Rue Madame 75006