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 following 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.
Please note that payments are managed directly by Shopify. It is Shopify, and not us, who charges you, and then transfers our part to us. For that reason, we cannot create or refund charges or control the billing cycle. We do not have access to your payment data either.
You may want to contact Shopify Support for any questions related to the app charges.