Add Gift Certificate to Checkout

Adds a *Gift Certificate Code* to *Checkout*. > #### Note > * *Gift Certificates* are treated as a payment methods. > * You are not able to purchase a gift certificate with a gift certificate. > * The rate limit is 20/hour (only for unique gift-certificate codes). > * Substitute your storefront domain for `yourstore.example.com`. > * The Send a Test Request feature is not currently supported for this endpoint.