I have been working on automated platform to reduce my ad costs and had a fairly successful pilot run with few agencies and sellers.
I wrote a blog post on being aggravated with amazon lusting for ad here
This is my product: AutoPilot
If anyone wants to give it a shot for their campaigns, message me. It's a free free trial (ie, no credit card or other nonsense)
We do this via 3PL warehouses in US & Europe r/https://www.aihello.com. However we have not integrated Walmart as none of our sellers have expressed any interest in it. I am happy to chat with you and help your business expand on our platform
disclaimer: I own and operate this site for my own ecommerce cross platform fulfillment.
There are softwares/plugins that does this.
you can checkout our blog: https://www.aihello.com/resources/blog/2020/04/13/integrate-woocomerce-with-amazon-fba-and-ship-like-a-boss/
If it's a new product it's hard to know unless you have at least one week of sales. For products with historical order you can follow what user strel0k has said.
I have also created and run my platform that does this for all skus => https://www.aihello.com
Shoot me a message if you need more information on this subject