Back glass repair

Not if it was broken by accident. If you purchased AppleCare + you can use one of your 2 damage incidents and pay the deductible.

Reportedly some of the better and more equipped 3rd party repair sites have a device that will let them replace just the back glass (Apple replaces the entire iPhone with a service replacement unit). You could call some repair sites in your area and ask if they can do such a repair and what the price is. If you do go this route Apple may refuse to provide any support for your iPhone in the future.