How do I activate AppleCare on AirPods?

This might interest you Applecare+ purchase at Costco - Apple Community

Assuming Costco is an authorized retailer of Apple products, they should register your purchase with Apple (Find information about your warranty or AppleCare plan - Apple Support). If for some reason they do not, click on this link to read how to 'Update your proof of purchase' --> Update your proof of purchase - Apple Support

To update your information contact Apple Support by phone. Tell the advisor that you need help with your Apple Limited Warranty or AppleCare plan expiration date.

Here’s what you need when you call:

- Your AppleCare agreement number

- The device serial number

- Your sales receipt

You just need to register your product with Apple online (Check Device Coverage page) under your Apple ID, and Apple knows that you've purchased your AirPods at Costco from the serial number. Once done, click your AirPods and it should have Applecare+ with the appropriate expiration date.