How do Indian merchants use this service when it’s not officially supported?

Hello,
Thank you for your supports and query.
You can find the supported currencies for Amazon Pay on below link
https://developer.amazon.com/docs/eu/amazon-pay-onetime/supported-currencies.html
So, If you have fulfilled Amazon Pay account through your business details and you have all those credentials like Seller ID, Access Key, Secret Key, Client ID etc.. then you can contact on my email clicksubhadeep[at]gmail.com

Thanks and regards
Subhadeep