I have to agree with this. Reading your messages, it sounds like your buyer is asking you to fix a bug they didn’t notice when you delivered their order. You then seem to say okay. However, you don’t say when you will fix this and instead try lecturing your buyer about what a great deal they have had from you.
It seems like you are misinterpreting your buyers’ request and motivations here. It also seems like you are angry about how much you are doing for how little you are charging.
If I was a Fiverr CS agent here, I would side with your buyer due to your messaging tone and you not fixing a bug you said you would.
In short, all of your stress with this order is coming from you. Sorry if that sounds critical, however, YOU are the one charging $30 here. You are also the one saying you will fix a bug FOC, rather than charge to cover the cost.
Overall, you are coming across as someone who has spent years working on Fiverr who is drained by the whole experience. However, this seems to be just your second order. In this case, you really need to take a step back and reassess everything from your communication to your gig pricing.
I’d say use the opportunity you have now to try and repair your relationship with this buyer. Fix what needs fixing and throw in something free or extra to say sorry for the communication breakdown.