No promotional service can guarantee book sales.
The Fussy Librarian's role is to help put your ebook in front of readers who are interested in its genre and give them an opportunity to discover it.
Readers still decide whether to click, download, purchase, read, review, or continue with other books by the author.
A promotion can create valuable reader exposure and activity, but the final outcome depends on the book, its retailer page, pricing, reviews, audience fit, and individual reader decisions.