Recently, I just saw a new feature inside the Amazon Ads console and it's related to Audience Optimisation.

So basically, under the “Audiences” tab, you can now increase bids for certain types of shoppers. That means
In Sponsored Products, you get 3 audience options:
1. People likely to buy
2. People who clicked or added your product to cart
3. People who already bought from your brand

But, In Sponsored Brands campaigns, it is available only for "new to brand shoppers" for now.
Have you noticed the same in your account? Let me know in the comments below.