I was in the same situation. First, I thanked them for giving me the opportunity to study at their school. Next, I expressed my surprise at how quickly they offered me a spot. For me, the response came in two weeks! At any rate, I told the school that I had not expected an official reply so soon and still needed an extra 7 - 10 days to sort things out. This worked for me.
You mentioned that you have been admitted to a few schools, so I'm sure that you will get an extension from one of them.
GMAT Club Premium Membership - big benefits and savings