If you are lucky enough to try this in your local store and find out the number isn't in use.... you should sign up for a new card with that number immediately. So many people use this trick that you will have rewards points coming out your ears. Just make sure you put down a real address so you can receive the many gifts and high-value coupons the store will be sending you.

Anyway this trick is more designed to get you the lowest price at a store that is an unusual stop for you, like if you are from the UK and visiting LV for a month. Or if you stop for a gallon of milk at a random store and don't feel like signing up for a card to get the sale price.