How do I get a full refund for an unintended Tezza Pro subscription on iPhone?

if it was me then I would try Request a refund for apps or content that you bought from Apple - Apple Support

otherwise reach out to their official support channels

Get Support (apple.com)

And or

Contact - Official Apple Support

And or

Contact - How to Contact Us - Apple

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 it over 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.

Report A Problem to ask for a refund; click here --> https://reportaproblem.apple.com/?s=6

Select:

Request a refund