My battery died over the weekend so I popped on over to Costco for a new one. I hereby retract my earlier recommendation. Their warranty is down to 36 months now. In my opinion, there is no longer any competitive reason to buy one from them. I would estimate that the price was about the same or perhaps very slightly cheaper than elsewhere.I've had warranty replacements on two Costco batteries-one in month 59 of the 5-year full replacement warranty they used to offer. WooHoo! Free battery! The second one was pretty far into the now-shorter warranty they offer, but the replacement still only cost me $12.
Anyway, in neither case did I have the receipt, but they just scanned my membership card, quickly found when the battery was purchased, and honored the warranty claim.