I just spoke to someone at USC about my lack letter so they're not closed for vacation as of yet but yeah call them so you don't have to stress about it any longer. Though it was funny, the guy I spoke to at USC, basically told me to e-mail him. Go figure. 😛
Last year I emailed them at 1:30am 3/7 and they got back with me later that day. Did you remember to include your name and file number in the email? Did you put a subject in the subject line so they knew it wasn't spam? I'm assuming you did, but it would explain their lack of response.