when we went to IOA 3 weeks ago, all the stores in WWOHP closed at 7 (park closing time when we were there) except honeydukes. the butterbeer/pumpkin juice carts outside started closing down about 5 minutes before 7, counting inventory and such, and wouldn't let me buy anything. they were pretty rigid about not selling me anything..."but i have money! i want to give you money. please take my money" haha
the guy at honeydukes told us they usually stay open after closing to accommodate the folks still in line and getting off of FJ ride. the gift shop at FJ exit, Filch's, stays open too, i believe, until the last person on FJ gets off.
so at least an hour after the park closes you can still shop at honeydukes depending how long the FJ line is at closing time. the honeydukes guy said they've stayed open as late as 9 pm before.