Effective immediately Apple will replace the battery in your iPhone 6 or newer device for $29, regardless of battery health.
This program runs through December of 2018. We can do the replacement at Capitol Macintosh; total cost with the battery and labor is $69 before tax. You should make an appointment for the service which takes about an hour to do.
If you replaced your battery at $79 before Apple's announcement, you may be able to get a refund by contacting Apple support.