There is certainly a case for considering the singular 'range' to be the subject, thereby demanding a singular verb. It's what you might call the mechanistic, formulaic view.
But our language can be subtler than that and there is just as good a case (in my view a better one) for considering 'houses' to be the subject with ''new range of' being an adjectival phrase qualifying that subject. Here's a case where 'range' is most certainly the subject:
"The whole new range of houses coming to Royston is smaller than that coming to Grantchester but is larger than the new range already built at Saffron Walden." Similarly:
"A number of cows in Farmer Giles' field are black. The number of cows in Farmer Giles' field is greater than the number in Farmer Jenks' field." In the first case 'cows' is the subject; in the second case 'number'.
Yes, it is the box of buttons which IS being delivered. That's what the intended meaning demands.
I'm getting as bored with this as some of the rest of you already are, but for different reasons. As a start, and as a goodwill gesture, I will now stop boring you. Have your "is" on me and enjoy it.