A very patient postal worker at my tiny local post office weighed and sized a stack of cards for me.
She explained when extra postage needs to be added:
If cards are embellished with products that 'pop up' above the paper(jewels, pearls, bows), the card can jam the sorting machinery.
An embellished card will need to be stamped or printed with 'Non-Machinable' across the front of the envelope. Extra postage will also need to be added, for example: if the card is one first class stamp weight, then just add a second ounce 20cent stamp 45+20= 65 cents.
If the card goes beyond 1 first class stamp weight, then add 2 second ounce stamps 45+40= 85 cents.
The size of the envelope also determines how much postage is needed. I was surprised at how large an envelope can be--6 by 11 inches. I was always putting extra postage on envelopes that I know now didn't need to be. However, square envelopes aren't so lucky, they need to be put into a regular sized envelope or add extra postage due to the sorting machinery.
Of course, if you still aren't sure how much postage a card needs, just visit your local Post Office for help!