Hi, welcome to the forums 
ChatGPT and the OpenAI API platform are two separate things. Very broadly speaking, one’s black and one’s white.
The white one, you have to pay what you use. the black one, you pay a monthly subscription.
The extension you’re talking about seems to require an API key, from the white platform. To use that, you have to add a payment method, and you will be charged for what you use (or what the app uses).
Your spending on the white platform has no bearing on your standing on the black platform, and vice versa. You can be a tier 5 white platform user and still use chatgpt basic.
In general, IMO, it’s not a super good idea to hand out your API keys to random apps - but that’s your business. If you want a subscription based AI pair programmer, I’d suggest github’s copilot.