Try laying an extra piece of cardstock over the front of your embellished card, and turn that part of your card toward the back of your envelope.
Most of my cards have ribbon, or buttons, or rhinestones. I am able to mail them this way using just one 44 cent stamp.
Also, since mail is fed through machines at the PO with the side with the stamp going through first, I try to position the card in the envelope so that any "bump" will not be on the corner of the envelope where the stamp is.
I mail lots of cards this way, and they arrive in good shape, and never are returned or "postage due."
I also just put my stamps on my cards and drop them the mail chute. Nine times out of ten, if you take a card to the counter, the clerk will look at it and say it needs extra postage.