Embed code throws warning/errors on both dev server and live site

On my dev server (localhost:3000) and on my production site (hosted by Vercel) I am getting either errors or a warning when using the href tag provided (aka the embed code). On my dev server, I'm getting these errors:
Access to XMLHttpRequest at 'https://checkoutlib.billsby.com/config/config.json' from origin 'http://localhost:3000' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header has a value 'https://localhost:3000' that is not equal to the supplied origin.
GET https://checkoutlib.billsby.com/config/config.json net::ERR_FAILED
Uncaught DOMException: Failed to execute 'send' on 'XMLHttpRequest'

On my live website, I'm getting this warning:
[Deprecation] Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience.

In both instances, clicking on the link does nothing else (doesn't bring up checkout, etc).