Oh gosh, I feel your excitement and if you can secretly return it it would be wonderful!
On one hand I would say don’t do it— and if they’re accepted then you can tell them in the moment of celebration that you’ll order them a sweatshirt and they get to pick it out (because so many schools have multiple sweatshirt options these days!).
But I’m a hypocrite as I did secretly buy a sweatshirt in advance because I saw a fantastic vintage one on eBay that fall, and I bought it as my kiddo had done the early financial pre-read guarantee with the school in July, and the school’s response was a number that was the highest merit award they offer “if they applied and were accepted.”
As it was the admissions officer for our region that made the merit pre-read decision, I took the number they responded with as a very solid sign that my student would indeed be admitted. I never told my kid “well, I think that’s a done deal” when the number came back, I just told them that it was great news, that the number put the school within the financial boundaries for applying ED, and that they should work hard and submit a strong application.
It sure was fun to give them that sweatshirt though when they opened the portal on decision day. 
Best wishes for the outcome you’re hoping for!!