Carrefour Belgium: Buy 3 boxes of LEGO, pay for 2

Belgian friends, it's your turn with this promotion at Carrefour Belgium: Buy 3 LEGO products, the cheapest of the three is offered to you (Amount deducted at checkout).

No limited-validity voucher or any credit on a loyalty card, the cashier will not count you the cheapest of your three boxes.
The offer is valid until December 3, 2012 and covers the entire LEGO range available in store.
(Thanks to uncle157 for his email)