How do you handle OpenAI billing for clients?

Hi everyone! I’m currently learning how to build simple AI agents (n8n + OpenAI) that handles emails and basic tasks for a client, but I have a question.

Since the OpenAI tool requires API credits to run, I’m curious about the best way to handle the billing. Obviously, the client should cover the usage costs, but how do you set this up professionally? It can be also related to VAPI, doesn't matter...

Do you usually:

1. Have the client create their own OpenAI account, add funds, and give you an API key?
2. Somehow Invite your email as a "Member" to their OpenAI account??
3. Pay for it yourself and bill them back with a markup?

I want to make sure I’m using the most professional and secure workflow. Any advice on what agencies usually do would be appreciated!

Option 3 as it keeps me under control and reduces support queries.

You need to build your own "credit system" by your own rules. Don't think like a developer and charge just "what is cost to me + X%" give your own packages to clients and make sure that you earn well. For example one email handle costs the client 1 credit or it's a part of 50 actions that you allow monthly.
Make sure that you earn at least (in my opinion) 100% because you give more value then just using openai api.

I bill them everything later - I extract usage and costs from OpenAI Azure and include it (tokens, etc)