Probe battery ****mers

I bought an AGM battery here for my Mercedes Benz c200 coupe last year February. A year later the battery is dead dead dead! They had said the battery has a 2 year warranty, when I get there a year later they tell me I bought a cheaper version and those don't last. I asked them why would they sell me a battery for R3000 that won't last Even 2 years ?! Since the difference between this better version and the one they sold me is R1000.