With today's electronically controlled vehicles, you don't need to get the exactly amperage battery to replace the stock one. I'm not sure what my G6 CCA rating is, but if it's say, 600CCA, you can install a 500CCA battery without worry. The only drawback will be if you constantly run an in car video system and keep the volume cranked up, while you run your 12V GPS unit, a 12V cooler in the trunk and have the cell phone plugged in, then, it MIGHT cause a problem as you'll draw more current than what the alternator can maintain.
Besides, I've found better deals on batteries, and even better brands at WalMart (out of all places), than at Costco. Be sure to shop around, a lot of the larger big box stores are competing with some Costco prices.
'11 Ram SXT 4x4 quad cab - 5.7L V8; 128,200kms
'09 Malibu 2Lt 3.6L - 140,000kms purchased 1 March 2019
'06 Pontiac G6 GT - 3.5L V6; 162,000kms (rear ended 15 Feb 2019, and given totalled title by insurance)