Jetpack Joyride

This achievement requires that you buy a matching set of clothing. A set refers to one body piece and one head piece (matching ones are adjacent to each other in the store). Though not the cheapest route, it is recommended you purchase the Top Hat and Classy Suit (13,500 coins total) as your matching set, as these are required for the "Class Act" achievement. As soon as you make the second purchase, the achievement unlocks. 

In the Clothing section of the Stash, there are multiple outfits that can be purchased. You must purchase both the hat and the suit of an outfit (listed next to one another). I would recommend purchasing the Top Hat (7,500 coins) and Classy Suit (6,000 coins) for the Class Act achievement.