I've been charged during the trial period

When you install the app, you need to accept the charge. However, you will not be charged until the trial period expires (this is how the Shopify standard trial model works).

During the trial, in the subscription section of the app, you will see the information below, which mentions exactly when your trial period expires (first bill date):

If you uninstall the app before the trial period ends, you will not be charged.

You may also receive a notification email from Shopify about your payment. Don't worry, that means that the payment has been approved, but you will not be charged until the trial period ends.

You may want to contact Shopify Support for any questions related to the app charges.