Changes to Track checkout started

added the optional property issues to the response with the 401 status

added the optional property issues to the response with the 500 status

the request property checkoutUrl became not nullable

the request property currency became not nullable

the request property items/items/priceCents became not nullable

the request property items/items/sku became not nullable

the request property items/items/variantId became not nullable

the request property items/items/variantTitle became not nullable

the request property totalCents became not nullable

added the optional property retryable to the response with the 401 status

added the optional property retryable to the response with the 500 status

added the optional property code to the response with the 401 status

added the optional property code to the response with the 500 status

the response property success became optional for the status 401

the response property success became optional for the status 500

endpoint added