yeah its always best to get the enclosesure set up a week before you pick up your new snake.
there can be all sorts of probs and extra things you may want to change that can be done before your new family member arrives.
Thats what i did and mine smashed the first mouse offerred after just three days in his tub.
Oh and P.s. Once a mouse has been thawed its best to throw it away if its not eaten within an hour or so. Re-freezing is not best practise as the mouse would have already started to decay.
Jim