I spoke with my post office and finally understand the cheapest way to mail cards.
 
You need to get the butterfly "non-machinable" stamp which costs $1.06 per stamp = ($0.66 forever stamp + $0.40 machine-able fee).
 
Here is a little table to understand how much postage to use:
 
Weight - Stamps
0 - 1.0 oz - 1 butterfly stamp1.1 oz - 2.0 oz - 1 butterfly + 1 forever stamps2.1oz - 3.0oz - 1 butterfly + 2 forever stamps