Apple probably thought your Airpod was damaged, not lost. Warranty and AppleCare+ coverage does offer reduced cost for damaged airpods, but lost ones you have to pay for yourself. For damaged ones you have to return the original.
If one or both of your AirPods or Charging Case needs replacement, read the "Damaged AirPods" section about a third of the way down this support document. It includes cost, which may be lessened by any AppleCare+ For Headphones coverage you may have, and a direct link to the support site for this service. Replacement instructions for lost items are provided at the bottom of this support article.
AirPods Service and Repair. click here --> https://support.apple.com/airpods/repair/service