According to Shopify app billing logic, when merchant uninstalls an app in the middle of app billing cycle, Shopify would still charge merchant for an entire month.
Note that app developers have no control over app billing as Shopify takes care of it for us. You can learn more about it in this Shopify document.
If that's not the case and you still think you are charged incorrectly, please respond to your support request with invoices and we are happy to take a look.