We're a non-profit courier service that wants to figure out the best way to integrate our service with Shopify. We have a number of current customers on the Shopify platform. Currently these customers are charging a flat rate when their end-customer selects local delivery. We want to simplify this process so that the end customer sees our live pricing. I've read through the CarrierService information but I'm still quite confused. Do we need a custom app built?