Many authors have never heard of Bookbub. The service is essentially a regular email offering selected discounted ebooks to a massive subscriber list. Most of the titles promoted therein are from major publishers, but a significant fraction are from independent authors and small publishers. Publishers and authors pay over $600 per title just to be considered for inclusion in their featured deals. They are extremely powerful in the world of ebook sales and massively profitable.
Peter Ralph has done a stellar job analysing the performance of bookbub and advising authors how to get one of the sought after featured deals. Other bloggers have useful posts about setting up effective ads for Bookbub, Others point out that while the sales spike created by bookbub is real and substantial, it can be rather short lived. This author suggests that the real benefit of being featured on bookbub is exposing the rest of your published work to a new audience.
In a world where bookstores, though gamely hanging on, represent a decreasing fraction of overall print sales (not to mention ebooks and audibooks), authors have to come to terms with the necessary techniques for online sales success, and letting the market know they even exist.