I replied "Happy Holidays!".
I prefer saying Happy Holidays than Merry Christmas because one you're implying that the person is celebrating Christmas what if they're celebrating Hannukah or Kwanzaa or they don't even celebrate Christmas. Happy holidays sums up all of that; Kwanzaa,Hannukah, Christmas and New Years.
I'm not a party popper or anything. Or a Mr.Scrooge I guess I just have a different way of thinking.