Glad to help. OpenAI Sales could do a bit better in communicating their service offerings.
Thanks for sharing the Expression to register your OpenAI API key; I was looking for it too :)
Also it saves you the trouble to reinstall a paclet as I (and StackExchange) initially suggested as a work-around.
SystemCredential["OPENAI_API_KEY"]="<YOUR API KEY>";
To check if your API keys have been registered use this Expression:
SystemCredentialKeys[ All]
Enjoy the OpenAI LLM in Mathematica.