If you uninstall an app, you’ll be charged a prorated amount based on the number of days you used the app for that app billing cycle.
Example:
You install an app that costs $9 per month on April 10th. You use the app through the 30-day free trial and on May 10th, you begin your paid usage of the the app. You use the app for another month and on June 10th, a $9 charge is added to your month-end Reverb statement (where you pay selling fees, shipping label fees, etc.).