Replacing 'AmazonSimplePay' with 'PayWithAmazon' on web site

My web site currently has AmazonSimplePay(discontinued). So, I'm trying to replace it PayWithAmazon. I just added a sandbox expressButton for PayWithAmazon code to my web page and tested, it seemed to returned back to my site and showed the transaction in the Amazon dashboard.
However, the AmazonSimplePay(that I want to replace), currently integrated on my website, takes the Amazon purchase and adds credits to that paying customer's web site account - in order to use those credits on my web...

Replacing 'AmazonSimplePay' with 'PayWithAmazon' on web site

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