I've been charged before the trial expiration
When you install the app you have to accept the charge. However, you won't be charged till 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 see a message like the following:
If you uninstall the app before the trial period finishes, you won't 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 won't be charged till the trial period finishes.
Please note that payments are managed directly by Shopify. It is Shopify, 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 don't have access to your payment data either.