When you’re building an integration for a specific merchant or not designed to be re-used across multiple accounts, the best integration approach is to use a merchant’s credentials (instead of building an app). Merchant credentials provide access to the REST API and JavaScript SDK, but cannot subscribe to webhooks.
Merchant credentials are only available on the Plus or Enterprise plan.
Using the REST API
To use the REST API with a merchant’s credentials, you’ll need to ask them to create an API key with the scopes that your integration requires (based on which API endpoints you want to call).
Once you have an API key, you can use it to make requests to the REST API on behalf of the merchant. Refer to the API Reference for a full list of available endpoints.
Using the JavaScript SDK
Using the JavaScript SDK on a merchant’s standard Shopify or BigCommerce storefront is simple, because the SDK is already automatically included and initialized. This means you can directly use the SDK’s methods and functionality, without any additional setup or asking the merchant for their credentials.
To use the JavaScript SDK on a custom storefront or other web or JavaScript surface, you’ll need to ask the merchant for their publishable key, and they’ll also need to create a signing key. You can then use these two values to include and initialize the JavaScript SDK. 
