i tried to set up amazon pay and get this error when test whether it works.
"Please add your redirect URI in the 'Allowed Return URLs' section under 'Web Settings' on the Amazon Seller Central App Console for Login with Amazon."
Does anyone know what the URL is?
I got the same error and it took me forever to figure it out but this is what I did and it worked for my store:
Then I went back to Shopify and clicked Deactivate Amazon Pay then Activate which took me back to the Amazon Pay setup. Although it said my account was completely set up, I clicked Next until it came to the Transfer Credentials button. I clicked the Transfer Credentials button and after a couple of seconds it took me back to Shopify Provider Payments.
I retested the Amazon Pay button and it worked.
I hope this works for you.