A point that unfortunately leads to a lot of confusion on the App Store: billing for apps is coupled with the billing cycle of your own Shopify store. This means that if you uninstall an app on e.g. the 15th of the month, Shopify can potentially bill for it up to 30 days later depending on your own billing cycle.
Example:
The trial started and the charge was accepted on May, 26 (7-day trial)
The trial ends around June, 02 >> 30 day billing period starts
Uninstalled June, 08
Shopify then bills for the app charges up to 30 days after the app was uninstalled
Here is the quote from Shopify's article on this which you can find here in detail.
Uninstalling apps with recurring charges
Make sure that you consider app billing cycles when you plan to uninstall an app. Recurring app charges are generated the first time an app charge is approved, and then on the first day of an app's billing cycle. Because of this, a charge will appear on your bill even if you uninstall an app only a day or two after you install it.