No Worries about that, as long as you don't say anything that will make the client report you, no harm will be done. Fiverr won't punish you for not closing the deal. Clients/Customer can always search for others sellers.
I think you shouldn't be worried if they report you because you are overcharging. It's about the quality and professionalism, I think.
Just tell Support You didn't do anything wrong they can review and see the messages on your inbox. If they see no harm was done. that the client just, want to get the hard work done for a cheap price then no problem for you.
You just Want a Fair Pay for Fair Service.