The first step to getting your story in ship shape is getting it critiqued.
Before shelling out any money on editing services that focus on your story at the sentence level, you need to make sure that your story as a whole works.
Maybe you've just finished your first draft and you want to get some feedback from someone external before sending round to your friends and family for feedback. Or maybe you've sent it round to all your friends and family already and you've received positive feedback, but want someone who's unafraid of giving you constructive criticism.
Whatever the case, a manuscript critique is a fantastic tool to highlight the strengths and weaknesses of your story, allowing you to fix problems with your story's foundation and putting you that bit further on the path to getting your book out into the world.