Dear user,
I think I can help you with your problem, I am reading now details of paymentwall webhook. I know shopify can be silly sometimes in not explaining well. I have experience in Shopify API and Shopify Webhook, but never worked with paymentwall.
I do not ensure you this will be a easy job. But in case I do not succeede, I won't ask you any payment.
In case you are willing to let me search for a solution, please contact me. I will respond in minutes.
Let's keep in touch,
SD