On October 1, 2013 Mlife extended their shopping discount to all Mlife Tiers.
Previously it had only been available to Gold and above.
Now, Sapphire level members get a 5% discount at participating retail shops.
Pearl and Gold level members get a 10% discount.
Platinum and Noir level members get a 15% discount.
Participating stores tend to be the lobby convenience stores and logo stores at MGM properties. When in doubt, ask if the store has an Mlife discount.

The Mlife Tier Benefits page has been updated to show this.