Store hours can be tricky in Paris. Aside from department stores, which keep slightly longer hours and usually shut their doors late on Thursday, shops tend to open around 10 am and close around 7 pm. It's not unusual to find a "back at 3" sign taped on the doors of smaller boutiques at lunchtime. Plan to do most of your foraging between Tuesday and Saturday, as the majority of shops, including department stores, are closed Sunday and some on Monday as well. You can find areas—particularly the Marais and tourist-oriented Champs-Élysées—where stores are open on Sunday. However, if you're making a special trip somewhere, always call ahead to check hours.
Galeries Lafayette and Au Printemps each offer 10%-off discount cards to foreign visitors. Some items marked with a red dot, usually designer clothing and sale merchandise, are excluded. To get a card, go to the welcome desk on the main floor of either store. Remember to bring a passport or driver's license.