If you want to make use of the T-Mobile US offer for Apple TV+, then you have to redeem as T-Mobile describes. The offer doesn’t apply until you redeem, and link your Apple ID account to the service through them.
With the T-Mobile US offer, the subscription fees will be paused for the duration of the offer period (depends on your plan). No cancelation should be done, when using the same Apple ID account.
Apple TV+ on US: Subscription included with your plan - T-Mobile (US)
Apple TV+ deal Frequently Asked Questions - T-Mobile (US)
↳ How do I redeem my Apple TV+ On Us?
If you think that you qualify for a refund, then you can request that: Request a refund for apps or content that you bought from Apple - Apple Support
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Note that Apple TV+ is just one of the subscriptions services. Other content in the Apple TV app, not marked tv+, will have to be paid for separately.
About Apple TV+ - Apple
Apple Originals - Apple TV+ Press
List of Apple TV+ original programming (episodic/series) - Wikipedia
List of Apple TV+ original films - Wikipedia
If you made a purchase for an item worth 6.99, different from the Apple TV+ subscription, then you are expected to pay for that separately. If you subscribed to some other service for 6.99, then you are expected to pay for that separately. Your card statement wouldn’t specify that. Your purchase history will detail what each charge was for. Please verify.
If you see 'apple.com/bill' on your billing statement - Apple Support
View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support
See your subscriptions overview - Apple