A manufacturer can absolutely legally force retailers to sell their products at specific prices. That isn't price fixing. Price fixing would be if Neptune and all their competitors all agreed to sell their automated Alkalinity testing machines for the same price.
You absolutely cannot directly force a retailer to sell a product at a specific price. Here is the FTC reference to this:
https://www.ftc.gov/tips-advice/com...ws/dealings-supply-chain/manufacturer-imposed
As a manufacturer you can set a MSRP and you can select who you do and do not distribute through. But you can not legally force them to sell at a specific price. Your only recourse if they don’t follow your MSRP is to drop them if you see fit.