Apple ID and Payment/Subscription Issue

Unknown Charge?

1. App Purchase
   1. Curious about a Charge? --> If you see an 'itunes.com/bill' or an unfamiliar charge on your statement
   2. If you have an issue with an app on App Store or billing --> Request a refund for an App Store or iTunes Store
2. Apple Card
   1. You may know your --> See your Apple Card spending history
   2. Also, you may want to --> Manage Apple Pay cards and activity on iPhone
   3. Report an unknown transaction or dispute a charge --> If you see an Apple Services charge you don't recognize on ...
3. Subscriptions
   1. You may view a subscription to an app --> See or cancel your subscriptions - Apple Support
   2. You can also request a refund for a recent renewal of an in-app subscription purchase 

Avoid accidental purchases

1. If it is your device then, please --> Use Guided Access with iPhone, iPad, and iPod touch - Apple ... before handing over a device to a child.
2. If it is your child's device then in the future, enable the Ask to Buy feature --> Request and make purchases with Ask to Buy - Apple Support

It could be Phishing

• Beware of --> Phishing & Other Suspicious Emails 
• Know how to --> Identify legitimate emails from the App Store or iTunes Store 
• Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams —> Use these tips to avoid scams and learn what to do if you think your Apple ID has been compromised.

Note that most of the links in the post by SravanKrA (many of which have no relevance to the situation as you describe it, so you do not need to read most of them) will take you to the Apple India version of Apple's documents. Typically there is no difference but there could be and I suggest you follow links I provide which will render the version appropriate to your location.

If you cancel a subscription you may have to do it at least 24 hours in advance of it renewing or you will still be charged. Have a look at: If you want to cancel a subscription from Apple - Apple Support

If there's an issue when updating a card, add a new card first, then remove the old card. If you need to change or update your Apple Account payment method - Apple Support

If you still have problems adding a card, contact Apple. Refer to this page for Apple Support features ➔ Contact - Official Apple Support

Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page under “Get more help” (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.

This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple Support - Apple Support For some countries look under “Other” listing at the end of the list. Not every country has telephone support, and phone numbers may only work when calling from the same country.