Q. All of this plus installation, at no cost to you. Is the comma necessary here? I think it is added sometimes to denote “and,” but then maybe an em dash would achieve greater dramatic pause or surprise.

A. The comma makes a difference. As you have it, with the comma, it seems to mean that for no cost you get “all of this and installation.” Without a comma, the meaning might be taken as “All of this [for a price]—plus installation at no cost.” Rather than depend on a comma, rewrite for clarity.