Ironware economics

A customer in a hardware shop asks: "How much do I pay for one?" "Three euros", the shopkeeper replies.

"And what about twelve?" is his next question, to which the shopkeeper answers: "That would be six euros."

"OK, then I'll take a hundred and twenty-one." the customer says and pays nine euros.

The question is: What did this customer buy?

Take your time to think before looking at the solution here

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