If you have a car, take a trip to the US Post Office in Ball Road. It's parallel to Katella Ave and near where the parking lots are for DL. The last time I was in the US and sent postcards was in 2008. An international postcard stamp was 92 cents. If you buy it from the PO, you pay the face value. If you buy it anywhere else, there will be a mark up. Be aware that even US Post Office Agents add a mark up.