Here's what I do:
Run a quarter sheet of any color cardstock through your crimper or corrugator.
(I believe Jen will soon be offering this tool at her awesome online store- The Coconut Scrapshop)
Place your corrugated panel in your envelope making sure it's covering the front of your card which should be facing the envelope flap.
Your design is cushioned under the crimped panel which adds a layer of protection. Cool, huh?
I can't take the credit for this tip. I received a card from someone years ago who had this brilliant idea. (I wish I could remember WHO!)
As far as mailing costs, I've found that it really depends on which postal worker serves me at the post office. I usually always take my cards to the counter and have them weighed and hand canceled.
Have a BLESSED day!