Can you buy a VMP membership when you decide to buy something?

stoopkid thought this was worth mentioning said
0

I know meh probably relies a little bit on people who forget they have a VMP membership and never use it like a gym contract. But I use mine so rarely. If I decided to buy something, could I first sign up for VMP for a month and then get the free shipping that same day? Or do they prevent you from using it on the item that is up when you sign up?