Their battery warranty should be better than autozones'. Go take the battery to them.
Just bring it to them
I don't know specifically what the return policy is for batteries purchased at Costco but I would like to think that if the battery is still within the experation date time frame that Costco would replace the entire package of batteries with no problem.
I don't think they pro rate batteries any longer. If the battery has a 3 year warranty and it goes bad they give you a new one
You can bring it back to Costco Tire Service and the service rep will pro-rate the return and sell you a new Interstate battery without making you pay a core charge.
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