Apple reportedly offers little help for AirPods owners once the batteries have worn out. It rather asks customers to purchase a replacement pair that costs almost the same amount as a new pair, which retail at $159, despite the companies claim that the batteries are "built to be long-lasting." There is, however, a way around this.
If your AirPods are less than a year old and the battery isn't delivering five-hours of listening time, Apple will replace them at no cost. You can also take out an AppleCare+ warranty for $29 that covers the battery, but only lasts two years.
If your pair are out of warranty, however, there is an option — providing you know the code words: "battery service." Then, Apple will replace them for $49 per stick; $98 in total, which is still pretty steep.
The reason for the maze in getting your AirPod batteries replaced is, according to Washington Post, because the headphones were never meant to be opened. In fact, they're glued shut. It doesn't seem there's any way to actually get the battery out without destroying the entire stick, which means the AirPod has to be replaced in its entirety, which is presumably why they're so expensive to fix.