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 stamp
1.1 oz - 2.0 oz - 1 butterfly + 1 forever stamps
2.1oz - 3.0oz - 1 butterfly + 2 forever stamps