There is one more way.. If you are using plugin from foxnetsoft and you are facing this issue then you can create a support request with them and check how they help you on this.
I have opened a ticket with them. And with the developer of the theme, and the developer of the Amazon plugin. But I am pretty sure it is not a plugin issue, as I can get any plugin to work on the second site successfully, but cannot get any plugin that re-directs a guest back to the site to complete Checkout (PayPal or Amazon Pay) to work on the first site.
Can there be a setting that limits the amount of guest accounts with items?