Uh, disregarding the other responder's apparent lack of financial concern, I would recommend writing a letter because it's unlikely to hurt you. It's not like the school will withdraw the offer of admission. As for a reason, how about 'though i'm very interested in your school, i haven't heard from every school, and I'd like to have all of the information in front of me before making a decision.' Likely to work? Yeah, I don't know, I agree that schools PROBABLY won't be sympathetic to this...but yeah your only other options are gambling or spending money that might be wasted.