For a private app, you will utilize the storefront access token from the Admin. This is different from the API Key and Password used for the Admin API.
For a custom app or public Sales Channel, make sure the app is first installed, then request a Storefront API access token from the Admin API using either theStorefrontAccessToken REST resource orstorefrontAccessTokenCreate via GraphQL mutation.
Checkout + Performance:
The Storefront API returns a Web URL field that your customer will complete the checkout process on.
It is best practice to only initialize a checkout when necessary.
If you are having issues with your .myshopify.com URL being surfaced at checkout, please check that you are initializing the client and API with the custom domain such as https://backyardbotanicgoods.com/api/2020-04/graphql vs. coolstore.myshopify.com/api/2020-04/graphql
The Storefront API is not designed to be used for flash sales for two main reasons:
The Storefront API has a limit on the amount of checkouts that can be created per minute. If an API client exceeds this throttle, then a 200 Throttled error 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.