The Payment for Stripe app collects a 1.2% transaction fee on top of your base Stripe fees for your country. In the U.S., those fees are 2.9% + $0.30 per transaction. There is no additional fee for manually entering card details. For example, the fee breakdown would be as follows:
$100 - $3.20 (Stripe fees) - $1.20 (app fees) = $95.60
In addition, a 1.2% fee will be applied to all Subscriptions created from the app.
These fees are outlined in the App Store description, in the Terms of Service you agree to when authorizing the app to connect to your Stripe account and on our website at https://paymentforstripe.com/pricing
You can reduce your Stripe fees to 2.7% + $0.05 per transaction by creating charges with a card reader, which you can purchase here: https://paymentforstripe.com/order