If a card is just a little lumpy, I will cut a piece of chipboard, and put in the envelope on the lumpy side of the card. It keeps the dimensional elements from getting caught in the postal machines. If it has some major big flowers, etc. on it, I use clear boxes. Google them. You can find them in all sizes. I just cut a white sheet of printer paper, and put the name and address on it, and put it over the top of the card in the box. Tape the ends shut, and the postage goes on the outside of the box. I've never had issues with them getting to their destinations in one piece.