App usage charges are associated with an app's 30-day billing cycle and represent charges for app usage events. So you used the app for 2 weeks and then canceled but are still liable for a month's subscription.
If you want to request a refund or credit for an app that you purchased from the Shopify App Store, then you can use the Get Support button to contact the app's developer. The developer might approve or deny your refund request, depending on the circumstances. Check the app's listing page or the developer's home page to see if they have a refund policy.
As the app charge was processed using Shopify, then the app's developer can issue you a refund or give you a credit. Credits will be applied toward any future Shopify App Store purchases. All credits that are applied to Shopify App Store purchases are documented on your subscription bill.