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, at the top of the app preferences page, you will also see a message like the one below, which mentions exactly when your trial period expires:
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.