They refused to replace my 6S battery unless I paid $150 to replace the screen (there was a crack outside of the main display).

Amazon sells a bunch of self-install battery kits for ~$21, which has a same-or-higher capacity battery, including the tooling.

It was very easy to do the self-install, and my 6S is just really snappy. This was only on iOS 11 too. Excited for iOS 12.