I think I've been charged during the trial period
When you select a subscription plan, Shopify sends to your email a notification of the charge approval. It can even appear in your "invoice summary" on your Shopify admin.
That message only means that you accepted the payment, but you won't be charged until the trial period expires. You won't be charged if you uninstall the app before the trial period expires.
You can see exactly when your trial period expires in the app preferences dashboard page: