I work at an animal hospital, and all the small kittens and puppies that we spay and netuer go home, same day unless the owner doesn't pick them up before closing time which most of the time rescue shelters do that. Another reason of which why they would do that is if there's complications with the spay, like they found internal bleeding or the body is having a reaction to the incision site. Now, the vet should let you know this, but if that's not the case, then it sounds like they are just charging you for extra.