From the official announcement
Summer afternoons are more refreshing with a free Starbucks® iced espresso. Buy any grande iced espresso beverage like an Iced Caramel Macchiato or bold Starbucks Doubleshot® on Ice, and get one free. Available June 27 through July 2, 2 to 5 p.m.
Free beverage must be of equal or lesser value. At participating US stores. Cannot be combined with other offers.
Example drinks that qualify: Iced Cinnamon Almondmilk Macchiato; Iced White Chocolate Mocha; Iced Caffe Mocha.
Pro-tip: Ask for “light ice” and they’ll reduce the amount of ice in your drink and replace that volume with more of the drink itself!