Mistaken purchase of an unknown app

Dear Sir/Madame,

I was notified that I have 4 viruses on the iPhone 8 Plus ;therefore, I downloaded an anti virus ‘McAfee’. Just after I inserted my fingerprint I realized it’s NOT free, so I deleted the app immediately ( I haven’t even opened the app icon , I didn’t use it at all). Consequently, I want to refund my payment; how can I do so ? When I opened the purchase history in my settings it showed $0.00 beside the app; I don’t know what this means. I hope you can help me refund my money as well as offering me help to protect my device from the viruses.

Thank you for the prompt reply.