Hey, Anh-Tho, just as a point of feedback, since you're leaving a lot of comments in this thread, it's not necessary to introduce yourself in each one. Most folks will be scanning the whole thread and seeing your introduction gets repetitive and breaks the conversational tone and makes it overly commercial. Thanks.
I was illustrating the point that customers are often unreasonable about billing, and there's no magical solution that fixes the problems. The answer is really to try to make sure you're not overly reliant on a single customer. That's hard as a startup though especially if you're in B2B.