Is there a special rate for using a card reader?

Yes! When you use a card reader, the Payment for Stripe app fee is 1.2% on top of your base Stripe card-present rate. In the US, Stripe's card-present rate is 2.7% + 5¢ per transaction. So a $100 sale looks like this:

$100 − $2.75 (Stripe) − $1.20 (app fee) = $96.05 net

You get these lower "card-present" rates instead of higher online "card-not-present" rates because the physical card is used at the reader — lower risk for Stripe. The fees are listed in the App Store description, in the Terms of Service you accept when connecting your Stripe account, and at paymentforstripe.com/pricing.