You do not say where you see this purchase. If it is an e-mail are you sure it is real and not an attempt to steal your private information ('phishing') (very common)?
Identifying legitimate emails from the iTunes Store - http://support.apple.com/kb/HT2075
Phishing & Other Suspicious Emails - https://www.apple.com/legal/more-resources/phishing/
Forward attempts to: reportphishing@apple.com
If you think the charge is real:
If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - https://support.apple.com/HT201382 - "Before you contact Apple to request a refund for a charge you don't recognize..."
If you use Family Sharing and you are the oganizer, make sure this is not something purchased by another family member.
Deleting an app does not cancel a subscription if there is one. You must cancel the subscription:
View, change, or cancel your subscriptions - https://support.apple.com/HT202039 - "Your membership or subscription might renew if you don't cancel your membership or subscription more than 24 hours before your renewal date." At this stage the renewal is considered a purchase. For a new subscription the sale is probably effective from when you first agree to subscribe.
Apple's terms of purchase in the USA are, "All Transactions are final" (https://www.apple.com/legal/internet-services/itunes/us/terms.html), but some countries have different laws so check terms for your country at: http://www.apple.com/legal/internet-services/itunes/. If sales are final then it is Apple's decision if they want to grant a refund. You can always ask, and people do get refunds on a case by case basis, but Apple can also say no. To request a refund:
Get help with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store - https://support.apple.com/HT204084
Report a Problem - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days"
Other ways to contact iTunes Store Support:
Click here, follow the instructions, and ask for a refund.
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Unwanted subscription is on my bill. How do I unsubscribe??
