Was this using Amazon Pay or selling on Amazon

I am using Amazon Pay on my Shopify store and am in this situation currently.

The customer messaged me asking if the product would fit his type of bicycle. I confirmed it would. Then he placed an order.

He chose DHL international shipping which costs about $45.

I shipped his order and sent him a shipping confirmation. A few hours later he replied to the SHIPPING CONFIRMATION saying he ordered by mistake and wants to cancel.

I told him orders can’t be cancelled once they have shipped. This is also clearly stated in the terms of service and cancellation and refund policies on my website.

Then 3 days later the customer contacted me via email stating that he decided the customs fees were to expensive and so he refused to accept the package.
He also rudely demanded a full refund including shipping and threatened to contact his bank.

I replied politely informing him of the policies and terms of use that HE AGREED TO at the the time he placed his order.

He replied threatening to tell his parents. (Seriously.) and said his dad was a lawyer and that he is going to post negative reviews online slandering my business.

DHL emailed me and asked how they should handle the package. 3 options: 1) I pay his customs fees 2) I pay return shipping fee 3) they dispose of the package.

Amazon Pay messaged me asking me to respond to his claim.
I have been looking for the cancellation policy imposed by Amazon Pay, but cannot find it. My terms and policies are very clear and every page on my site has links to these terms/policies.

Can someone tell me if Amazon Pay will recognize and respect the terms of use and policies of my business?

Or can someone point me to where I can read the terms Amazon forces sellers to be subjected to?

Thank you.