Can I charge a fee for my app once it's in stores?

With iOS, you can choose to release your app for free, or charge any price going from $.99 and above in tiered increments pre-determined by Apple.

Beyond free apps, Google Play allows you to set pricing from $0.99 to $200 USD:

Google Play Prices and Supported Currency

To set your price in the Shoutem builder, visit the Publish Info tab, and set your price through the dropdown menu.

Note that the Android application can't be set as Paid if you decide to publish it under Shoutem, Inc Google Play developer account. 

Also, on Google Play store, once the app is a paid app you can't change it to a free app anymore.