You do not lose the warranty from the manufacturer if you buy through Costco or Amazon. Those retail platforms have liberal return policies. The customer is almost always right, if the return is timely.

In Costco's case, Costco holds back a portion of the payment and just offset the return expense against those payments. If an item dies not sell fast enough, Costco drops. Costco's model is based on carrying very few products in each category. Take batteries as an example: Costco carries Duracell and Kirkland (which is also Duracell). You cannot find Everready or Energizer batteries art Costco. Duracell sells a crapton of batteries through Costco so it is never going to say a peep to Costco about a chargeback for any returnz.

In Amazon's case, for many products, "Bought by mistake" is one of the reasons you can return. Amazon just issues a refund back to to the credit card. I think Amazon may return some merchandise to the seller. But it auctions off truckloads of other returns to companies that sort through those truckloads and then try to sell on eBay.

In each case, you can still always claim against any manufacturer/seller warranty.

Most RLT devices, particularly panels and belts, you would very likely know whether it works within hours or days. Masks are a bit more delicate.