For bulbs that are really cheap, people won't keep their receipts and warranty information. I think it will be hard for bulbs to differentiate by warranty.
If you buy at Costco, you can return them at any time in the future without a receipt. The return desk will lookup past purchases using your membership card.
yea if it fails after a year (vs. a few days) very few people are going to bother with or even remember there was a warrantee
The advantage of buying online.
Or Home Depot. Give them your email address and attach your cards to it and then you will always have the receipt.