Hide Billing/Shipping sections in WooCommerce when using Amazon Payments

That extension should use its own forms and not the default ones:
https://docs.woocommerce.com/document/amazon-payments-advanced/#section-7

You should be able to create a ticket for specialized help with that extension here:
https://woocommerce.com/my-account/create-a-ticket/

Yes, the Pay with Amazon does use its own forms. It injects them using iFrames. I would like to conditionally hide the other stuff – the standard/default stuff since the customer doesn’t enter a billing/shipping because that is based on the user’s Amazon account.

I’m surprised the extension doesn’t do that by default. May be due to how the checkout page could look with all those fields hidden.

You can contact WooCommerce.com support about this, and suggest the idea. But the customization is outside the realm of support we can provide here.