I could not get the module to show the headers. If the module is installed dont think you need to add the custom header. I tried the following but did not work. Lot of people experimenting but no one has answers. :/ Maybe you have better luck.
<cors enabled="true">
<add origin="*" />
</cors>
I bet you are using one page chekout. I had the same issue. It has to do with form validation where some fields are required but doesnt stop the payment plugin from pushing through the paytment. Fox sent to me a beta to test v3.07 which is supposed to check if the form is validated, however, I opted to get rid of one page checkout due to many things going on at this step. By the time users get to the payment page, I want them fully validated to make the payment plugin work less. Hope this helps.
I am thinking about testing this in IIS10 for the same issue. What configuration did you use?
Going to try the following.
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<system.webServer>
<httpProtocol>
<customHeaders>
<add name="Access-Control-Allow-Origin" value="*" />
</customHeaders>
</httpProtocol>
</system.webServer>
</configuration>