Thanks for your detailed explanation, we really appreciate the time, you gave to debug and write that detailed explanation, it really helps us in understand the issue properly.
I have tried to find the cause of the issue and found that when I activated the WP default Theme Twenty Twenty Two on your site, the Place Order worked.
And then I tried to activate the XStore Parent Theme on your site and the Place Order button worked: https://postimg.cc/HVvGNrNf
So it makes clear that the issue is with your Child Theme Customization only. If you did any Customization in Child Theme then try to remove it and check back OR try to delete the Child Theme and Install the Fresh new one and check back. Hope this will resolve your issue.
Regards 8Themes Team.
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