You can refill your mug at the bakery or the pool bar. But why buy a mug? You'll have a full size refrigerator, it would be cheaper to buy a liter of pop/soda even at the inflated Screen Door price. There's a coffeemaker in the room. There's a Hess station across the street that sells convenience store items at a good price.
You'll love it! But beware...you'll never want to go back to a regular hotel room!