Why should I pay for a replacement AirPod after the right one dies after 8 days of use?

WHO told you that?

For any undamaged product, simply return it with its included accessories and packaging along with the original receipt (or gift receipt) within 14 days of the date you receive the product, and we'll exchange it or offer a refund based upon the original payment method.

Returns & Refunds - Shopping Help - Apple

They certainly are covered by warranty for manufacturer's defects

Apple One (1) Year Limited Warranty – Accessory - For Apple and Beats Branded Products Only.

Did Apple give any reason for declining to replace the unit? A warranty would cover an early failure of a unit due to manufacturing issues, but not some accident of your own making such as might be caused by a significant drop to a hard surface, treading on a dropped device, or exposure to liquid. Did you run through the advice in Get help with your AirPods - Apple Support?

tt2

You should not have to pay unless the issue stems from damage sustained by the airpods or user caused problems.

You can make use of the warranty and get them replaced. If it is found to be a manufacturing defect it would be free.

If it was something else, not attributable to the manufacturing process, then it would outside the scope of the warranty.

Who said you had to pay? Why did they say you had to pay?

Hmmmm... how a person that cannot "inspect" the actual device could say that can only meant that you admitted to damaging the Airpod to her...

Did you save the Chat session transcript?

Take option 2

You didn't mention THAT before

- no one "online" can evaluate a device, and they will not "take your word" for anything

It is moot now that you have revealed that she told to take it to a Store for evaluation

So what did they say at the store when you returned it?

Why did they not refund you your money?

Is the Airpod actually damaged?

Well OK then... problem solved.

Have a great day.