I, too, have sent several postcards through the mail. I've never paid any extra postage for normal postcard size.. I do, however, like to have them hand-cancelled and put in a cellophane (see-through) envelope. (Also for those that are really special or have crazy quilting or beads, I ask to have them hand-cancelled and then put in a regular envelope. I know that sounds silly because I pay the postage twice! I did one this way for a friend who had just taken her citizenship oath (having it cancelled on the day of her ceremoy) and she loved it.
All that said, the first thing I should have written is that your card is very special, nice colors and your mother should be thrilled with it.