Hi rodolfo,
you can do this by using weight
Set all products apart from the 1 product that costs 6 to a weight of 0,00 or 0,01
Set the weight of the 6 product to say 100,00
Then set up the shipping buy weight:
Product weight order to 99,00 cost 3,5
Product weight order to 100,00 and above to 6,00
i cant remember exactly how to do this but using weight is way to go.
Play about with it and you should get it to work
I hope this helps you out
Geoff