403 error trying to return to site after Amazon purchase off site

Using IThemes Security 4.1.3 on WooCommerce site hosted on Hostgator.

Purchases on the site using WooCommerce Amazon Simple Pay plugin work perfectly if Ithemes Security is disabled. When Ithemes security is enabled, purchases go through. But when I clock the button on Amazon to return to WooCommerce site, I get a 403 permissions error.

In Ithemes Security, I tried deactivating two protections:
*Long URL strings
*Suspicious URL items
I then cleared server and browser caches. I still got the same 403 error.

1) Are there other specific protection settings in Ithemes that need to be deactivated?
2) Is Ithemes Security inherently in conflict with Amazon payments?

Thank you.

https://www.ads-software.com/plugins/better-wp-security/