Another place to find professional editing service is the team at Ebook Launch. They have a team of professional editors who can polish your book for an affordable price. You can even get a free sample edit of 750 words to help you decide if they're a good fit. The small team at Ebook Launch also provides amazing book cover designs and formatting in addition to their editing service.
If an editor’s rates seem skeptically low, send an editing test, ask for references or a sample, place a hidden message in your post, and see if they understand the different types of editing. If an editor passes all of these tests, even if she’s a newbie, give her a try. This could save you hundreds of dollars and help you find a skilled editor who is competent and affordable.
Next, I check their work history to see what kind of projects they have been working on. For example, if I’m hiring an editor and I see that 8 out of their last 10 projects had nothing to do with editing, I will probably pass because it means they either don’t enjoy editing work or they aren’t able to find enough of it on Upwork, which should not be a problem for a highly skilled editor.
Not that I’m not looking forward to seeing the author’s work. I am. Seriously, I am! Book editing, despite the sticker shock first-time authors often get when they hear the estimate, is not a profession anyone enters in hopes of getting rich. I truly love helping a writer make his or her work the best it can be, and I’m always eager to get started. But I realize the task of writing takes time, usually more time than the author (even many an experienced author) expects.
From your comment, you may be in search of a manuscript critique, where you pay a professional to essentially conduct a beta read with the added benefit of their experience and knowledge about the craft. Then they’ll give you a few pages’ worth of considerations. When I don’t have the time for such critiques (and I only critique nonfiction), I refer my clients to andilit.com.
I like the idea of embedding “fine print” into the initial email. I also think it’s a good idea for an author to check references; the questions Val suggested are great. Third, as another commenter said, an editor can give the author a general assessment of their book, which will help the author decide if that particular editor will be a good fit or not.
Regarding your last question, I know of no editor who gives a money-back guarantee for occasional errors that happen to slip through or for a different opinion by a later editor. I’m afraid such things are just the nature of the publishing beast. But by having a shorter portion edited first, you will get a decent feel for whether someone is the right editor for you, and will be able to be confident in his or her work on your entire book.