Welcome to the Custom Storefronts community board. Please read through this information prior to asking your question.
What is a custom storefront?
Bug Reporting + Feedback:
StorefrontAccessTokenREST resource or
storefrontAccessTokenCreatevia GraphQL mutation.
Checkout + Performance:
200 Throttlederror response is returned. Shopify recommends designing your app to be resilient to this scenario. For example, you could implement a request queue with an exponential backoff algorithm.