Polaris is an excellent frontend framework with very good documentation (unlike Shopify's API docs!), but there's one thing in particular they don't document very well: how to get working propertly with React routing packages like Navi or React Router.
Learn how to check if a discount code is valid from the storefront, without making any calls to your own server or app.
Sometimes your Shopify app grows beyond its original scope and you need more OAuth permissions -- or perhaps Shopify has changed the behavior of one of the permissions you were using and suddenly you need to ask for a new permission to continue working. Learn how to check whether your existing users need to update their permissions in this post.