I think it is unfair

Packets of coffee on special. Marked 2 for R140. So I ordered a case of 12 online. Once I print the order confirmation I see I am not charged R840 but R906. I phoned and queried. The idiots tell me i should not have bought a case but rather 12 loose packs. Can some one please tell me what it is that I am missing, what is the difference?