From the FAQ [1], too:

>> Can I purchase more credits?

> We currently don’t support the ability to purchase more credits on a one-time basis.

> If you are on a ChatGPT Plus and would like to access more credits to use with Sora, you can upgrade to the Pro plan.

Ouch. Looks like they're really pushing this ChatGPT pro subscription. Between the watermark and being unable to buy more credits, the plus plan is basically a small trial.

[1] https://help.openai.com/en/articles/10245774-sora-billing-cr...