Cant find where Shopify sends the API request to our app to get the shipping rates

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We have an app (called Snooze Ecom Service) which provides the shipping rates (for example, PICKUP -> free, Standard Delivery -> $80, Next Day Delivery -> $90) to our Shopify store.
There is a bug in the system that allows customers to pay POA orders (When an order is a POA due to the far distance between the closest store and the shipping location, the customer should not be able to see the payment form and pay it.)
I'm trying to debug it, but don't know where that API is called from Shopify to our Ecom app.
I appreciate if you could help me to find where is the Shipping rate API setting for Ecom app is in Shopify.