Accidental purchase
How do I return a music album I bought accidentally
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Strictly speaking all purchases are final: please see this page for information on requesting a refund: Request a refund for apps or content that you bought from Apple - Apple Support - note that just deleting an app doesn't cancel the subscription. (To cancel a subscription please see How to cancel a subscription from Apple - Apple Support)
However Apple have been known to be sympathetic in cases of actual error, so it's possible that they may allow you a refund as a one-off concession: go to https://reportaproblem.apple.com/?s=6