It’s technically prohibited by OpenAI’s ToS to share your key with any third party, so presumably options 1 and 3 could get your users’ accounts banned. That being said, until OpenAI releases some kind of OAuth, there isn’t a great option unless your app is entirely client side.

Why not use your own api key and collect payment from your customers?

or add advertising and make sure you use 3.5

Definitely local storage.