on Self Publishing Advice:
Why Run Ads on Amazon at All?
Advertising a book on Amazon is one way to help ensure readers have a chance to see it when they are looking for similar books on Amazon. Most books are simply never seen by readers, as there are now so many on Amazon.
But first, a safety note. Please test Amazon ads with a small daily budget perhaps $2 a day with 3 different ads (campaigns) = $6 total daily budget, but only if you can afford to lose this for a few days or longer.
It can take a week or two for Amazon to start showing your ads, depending on your bids and genre, so if nothing happens in the first few days, please be patient and keep monitoring. The ads can start at any time. They usually start slowly, spending far less than your daily budget, but in some smaller genres the ads start fast and will spend your full daily budget.
So, monitor the results every day to see what your spend is. And monitor your Royalties in your KDP dashboard.