How can I add a custom payment method to my Shopify store?

[Skydescent]

I'm trying to create a custom payment method for Shopify. Problems start with the fact that to create a payment extension you need to be an approved Payment Partner . But as Shopify support said me, the payment partner approval process is currently on hold until Shopify developers finalize the new payment API. It is not known exactly when this will be done.

I found a couple of solutions when custom Shopify app Admin API key  is passed to the payment service and custom javaScript is added to Shopify store -> Settings -> Checkout -> Order status page -> Additional scripts. I used this example to understand:

<script>
  {% if checkout.shipping_method.title == 'Pick-up at the store' %}
    Shopify.Checkout.OrderStatus.addContentBox(
     '<h2>Pick-up in store</h2>',
     '<p>We are open everyday from 9am to 5pm.</p>'
    )
  {% endif %}
</script>

Ok, i added this script, but still default method title is shown:

 

Could it be because the Shopify store is on test mode?

If anyone has encountered similar problems, please suggest possible solutions?

Very grateful for any help, thank you!