Save your AirPods’ serial number to replace lost parts
"Take a screenshot or picture of your AirPods serial number. You can’t replace any parts without it.The Full StoryDo this when you first get them!!
You can take a picture of the box or a screenshot from Settings > General > About > AirPods so that Apple can pull up the serial number.
If you lose the case and don’t have the serial number anywhere, you can’t even replace the case without it."