Hi there.
I have created an extension-only app, which uses the payment customization function API. It works as expected in my personal dev store.
Transferring it to my work (a Plus organization) is proving tricky. It has installed fine as expected, via the install link. However, when i go to Settings → Payments → Payment Method Customizations → Add Customization, and select the installed app extension template, it doesn’t activate the function. Instead, it redirects to the app settings page (the URL is /settings/apps/app_installations/app/cod-checkout-script).
Both Support and Partners Support have looked and said, “this looks strange, we’ll be in touch”.
Just wondering if anyone else has faced a similar problem activating an extension?
Thanks in advance 
Okay, I managed to figure it out. It turns out that, in order to activate the function, you need to run the GraphQL mutation to create the payment customisation object for EACH store you install the extension.
I had run it on my dev store and created it there, but it also needs to be run via GraphiQL for each installed store instance too. Just needed to install the GraphiQL Shopify app and run it there. Boom! Done. The docs are a little murky in terms of using the CLI on dev stores and production stores, and I was struggling to login to my work org with it. Support kindly informed me the CLI would only work on “Partner created development stores”, hence GraphiQL to the rescue 
Hopefully this might help anyone else struggling with this.
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Hello. I actually need help for the installation of a customisable payment link api. In my country, there’s no payment gateway that answers when you submit to them. So I apply with a payment gateway that does not partner with shopify which went well. But now I am stucked at getting the payment gateway to shopify. If anyone can help