You can use the coupon FREEEMAILSELECTOR to install the app for free.
1. Go to https://www.zendesk.com/apps/support/reply-to-email-selector/
And press Buy
2. Click Add coupon code
3. Enter Coupon FREEEMAILSELECTOR
and press Apply
4. Press Proceed to payment.
Keep groupMapping field empty.
The app will be installed. No credit card is required.
5. Open any ticket to test and configure the app