4. When buying expensive Champagne, you should ask if you can have a bottle from the restocking room.
When buying Champagne, especially those in a clear glass bottles (like Ruinart), you should ask if you can buy one from the store's back room rather than from the shelf, as Champagne starts to degrade in quality when it's exposed to light (hence, Champagne caves) so buying it straight out of its shipping box will ensure a higher quality.
[Photo via @moetchandon]