I shop at Aeropostale a lot, and I am also a former manager for the company so I'm familiar with the signage for the stores. There are signs for just sweatshirts, and there are signs for just pants. Usually on their signs, the front will say 'Sweatshirts' and the price, and then the back will say 'Pants' and the price. There are not signs for both, nor does Aero sell the sweatshirts/pants as a set. They do occasionally run items 2 for $25/30/35 etc, and if the items are both 2 for whatever amount, you can get them as a set then.

Next time, just ask if theres a question on pricing. Or if the associate rings you up and you think the price is wrong, say something. Don't just go ahead and buy the stuff, then complain about it later. They didn't force you to buy both of the outfits, you made that decision on your own.