You are a seller. Amazon makes up 40% of your sales. You have your own first party website where you can profitably offer your product for 5% less and still make an additional 10% profit but it only makes up 5% of your sales currently.

What's your move in this position?

You can't just imagine all companies as perfect frictionless spherical interchangeable cows. In reality companies as large as Amazon can have significant market distorting effect, while much smaller companies might have to choose between their principles and going out of business.