Hello!
I also got accepted into University of Pittsburgh for Fall 2013! I got the same acceptance letter in the mail with the "Pay by Jan. 21" sentence. I called Philip Mack who I believe is the Director of Admissions, and he told me that it was a mistake to have the Jan 21st date on the letter, and that I did not have to send in the tuition deposit until after I got my scholarship letter on Feb 15th, and I had until March 15th to put in a deposit. I'm relieved, because I was really nervous about putting in a deposit before I knew if I could afford Pitt. I hope this helps!