iPhone 15 battery health at 89% after 278 cycles

That’s probably a little on the low side, but depending on your charging methods and usage, is inline with the iPhone’s age. If you haven’t reviewed the Apple support article I linked below, you may want to take a few minutes and review the information.

Charge and maintain your iPhone battery - Apple Support

Also at 1-2 years old and presumably out of warranty you have no way to be compensated. Enjoy the next 1 to 2 years of happy usage then have new battery fitted by Apple