This seems like a very basic web 1.0 request Squarespace to be able to search your own site and not jus blog posts.
I just built a huge site that needs a search tool and now i'm left needing to pay extra to some anonymous 3rd party?
Hire those guys who build the 3rd party plugins for some help, if they can do it how come you cant?
Squarespace costs a fricken fortune these days, come on!