How do you cancel a PENDING purchase made in the APP store by mistake?

The purchase is not pending. Purchase has been completed in the App Store. What is pending is the payment. This pending payment will post (complete) in a few days and then you may request a refund here ➔ Refunds, Subscriptions & Billing Questions

Axel F.

You can request a refund from her:

https://reportaproblem.apple.com

If you can not choose a refund, because it is written as "Pending" - then wait until the purchase is completed, and then request a refund.