Preserve the product. Everything else is secondary.

Why they ask, and why the request is reasonable-sounding

The manufacturer will offer a refund, a replacement, and a prepaid label so they can investigate. Once the product leaves your possession you lose control of how it is examined and whether it comes back intact. Any legitimate examination can happen under an agreed protocol with both sides present.

Spoliation cuts both ways

Spoliation is the loss or destruction of evidence a party knew or should have known was relevant to a claim. If you lose the product after you knew a claim existed, a court can penalize you — by instructing a jury to assume it would have been unfavorable, by excluding your expert's testimony, or in bad cases by dismissing the claim.

What to preserve, specifically

More than people expect, and stored deliberately rather than in a garage where it gets moved.

• The product, in exactly the condition it was in after the failure. Don't clean it, repair it, or take it apart
• Every fragment, including anything that broke off
• The packaging, manual, warnings, inserts, and any warranty card
• The receipt, order confirmation, or card statement
• The model number, serial number, lot code, and date of manufacture — photograph the label
• Batteries, chargers, cords, adapters, and any accessory involved
• Photographs of the product and the scene from every angle, before anything is moved
• Any related communication with the manufacturer or retailer, including chat logs

How to store it

Somewhere dry, secure, and undisturbed, where it won't be thrown out by someone else. Photograph it as stored, note the date, and keep a simple record of everyone who has handled it since the injury — that's chain of custody, and it forecloses an argument about whether the item examined is the item that failed.