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:

Have the client create their own OpenAI account, add funds, and give you an API key?
Somehow Invite your email as a "Member" to their OpenAI account??
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!

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