"The problem is that the Costco/Interstate site says they donít have the correct battery, and the guy at the Costco auto desk knew nothing about the battery or its application."
The Manufacturer specd an AGM, if the Costco battery is not an AGM type, don't buy it.
Penn-Deka makes an AGM in that group size: http://www.dekacatalog.com/
You can call or email Hensley in Greeley for a price:
Last time I bought an AGM for my vehicle, Rocky Mtn. Battery gave me the best price, and had it in stock the next day: https://www.rockymtnbattery.com/