Here's a tip about receipts that was suggested to me by a store employee: make a copy of it. Most stores now print receipts on thermal paper, and it will fade over time, potentially making it useless if you need to redeem a warranty on a battery. I like to make a copy and keep the copy in the car, and put the original receipt in the file folder I keep at home for each car.
I couldn't beat the price and warranty on the Energizer battery (group 51R) at Sam's for my Honda, and curiously, the battery for my El Camino (group 75), which is physically much larger, is $15 cheaper. I'll probably buy one in the spring.