We have a shopping website where we sell products. We want to push data(orders, products) from our system to our Shopify store. Also, pull products inventory and other information from the same Store. Earlier we want to create a public app but as per guidelines we need to use the ShopifyBilling API. So now we are thinking to use the private/custom app. will the requirement to implement the Shopify Billing API applicable for private apps as well?? Is it mandatory to use the Shopify checkout to sync the orders?