As a self-published author, learning about marketing has been an eye-opening experience. There are a lot of options available. Facebook promotions for this website were the first I tried. With $10, I reached 773 people and got one person that clicked on the ad. Facebook had estimated that the post would reach 1300-3000 or so people, so its estimate was a bit off. Also, there's no way to know if that one click resulted in a sale. Not exactly a great result, but perhaps I need to better target my audience. I'll keep experimenting!
Amazon, on the other hand, allows for two basic types of paid advertisements. You have to be enrolled in KDP select to use them:
Product display ad:
Minimum $100 budget. Display ads on Kindle homepages, product searches, etc. Cost per click is usually $0.15 to $0.20, and you're only billing for the clicks you get.
Sponsored products ad:
Displays ads on search results and product detail pages. You can choose a budget per day and how many days to run the ad campaign. Again, you're only billed per click.
I chose to run both. Amazon keeps track of the sales from the campaign, so you can see if the campaign is actually profitable. It's pretty robust, and I'm interested to see the results!