This is a really tricky situation that you're in, and it's hard to know what the best course of action is. It sounds like this customer is taking advantage of you and your kindness in agreeing to do work for them in the future. It's definitely not right for them to be blackmailing you like this, and it's not something that you should have to put up with. I had an experience of being extorted, and some experts helped me. Maybe they could help you too. Also, the best thing to do in this situation is to stand your ground and not give in to their demands. Suppose they follow through with their threat and contact Fiverr. In that case, you can explain the situation to them, and hopefully, they will understand. In the meantime, it's probably best to avoid this customer and not take on any work for them, even if they offer to pay you.