Shops, much like rides, close fairly quickly once it hits park close time. If the park close is at 7pm, shops try to close by 715p. Shops attached to rides tend to stay open longer, depending on if the ride closes with a line or not.
Shops towards the front of the park stay open longer, with the main shops (Universal Studios Store & Islands Trading Company) staying open latest. As with rides, as long as you are *in* by closing time, then you get to shop.