Should have split the stuff up. So instead of spending E90 and they end up only giving you one free coke, then split into three E30 portions and pay for each one at a time. Technically they should then give you three cokes. If they push the one per customer idea then phone up friends and family to slowly go through their checkout with exactly E30 of stuff. And pay by cheque. Slowly. After putting lots of stuff back and forgetting what you were doing a lot...
Seriously though, they should have acted a lot better. Screaming and throwing things around is hardly nice, especially when it is the people running the place. Some people need to relax before it kills them (or someone else does who isn't as mentally adjusted as the rest of us and takes offence).
Cheers