Getting the Discount Coupon is really easy:

• Log into the Pokemon X and Y Global Link.
• Look for the Pokemileage tab.
• Click on the Discount Coupon tab and spend 1 Pokemiles.
• Click on the Download the Discount Coupon button.
• Then Game Sync using the PSS so the item is sent to your game. You can pick it up from the delivery girl at Pokémon Centers around.