Open ai free api for a web app?

I want to get a gpt of gpt store and turn it in a web application.

I'm a newbie to this topic and if I'm not mistaken I need an open ai api and every time someone uses gpt from my website do they charge me money?

Is there a way to do it for free?

Yes they will charge you. There are free apis but you have to handle the limitations.

To make enterprise level stuff you should calculate your api costs and pass them to the user.

The free apis come from reverse proxies, they do have a limitation like Matheus mentioned

Openai and claude are the major ones will charge you per api call based on tokens used. They will return you the cost in each api response. You can handle it at your app level.
Both are now prepay if i am not mistaken claude is for sure, min deposit is 5$.
Groq ai is free at the moment, you can use the api fro free: console.groq.com/login