Hi there! Could you give us a little more information on what you’re trying to do? It’s hard to guess at why the error might be happening without more information. For example: what is the intended use of the post purchase extension? What is the structure of the changeset you’re trying to apply?
In this thread someone had a similar issue that might’ve been because of CORS errors and using webhooks seems to have fixed their issues. Could that be what you’re running into?