We use product variants to add an "Order Protection" product to the customers cart, if they opt in to the protection. Although all the shipping protection apps on the Shopify app store use product variants to add the additional fee, we have been rejected. This is the response from the reviewer:
"Since the order protection is being added on our checkout page and the app is adding a variant products it bypasses our shipping rates for the products. We can resolve this by changing the functionality of the app, to just add additional cost only and not by adding a product on the merchants store. "
It seems like they want us to use a different approach to add the additional fee.
We looked at using these two endpoints instead, however they wouldn't work since the Order Protection fee would only be added at the time the order is placed, meaning we would not know whether the customer had opted in and whether we should add the Order protection fee: