I did a similar thing in 2003 -- codes from 200 chocolate bars could be collected for a free xbox (5GBP shipping, normal cost 160GBP). This coupled with a 5 bars for 1GBP offer in a local supermarker meant I got an xbox and 200 bars of chocolate for 45GBP.
The even more famous one was Hoover, who were offering free flights to the USA if you bought a vacuum cleaner.
They were shocked(!) to discover people were buying 100GBP appliances to get a free flight worth 500GBP.
Then they decided to get 'creative'. They offered people flights that involved multiple inconvenient changes, or gave passengers in the south of England flights that left from the North of Scotland and v.v. They set up fake call centers who would ring customers up and tell them that the flight had been changed and send them to the wrong airport, they would send others long complex legal forms to sign which in the small print actually waived the prize.
The result after paying out a few million quid in tickets, several 100,000 in fines and a lot of annoyed customers - is that 20years later the company that was the generic term for vacuum cleaner in the UK is now known mostly for the con.
Link in Finnish, but scroll down for pictures: http://www.pulu.org/pro/1-1500-litraa-pepsia-ja-sony-vaio-s5