Mailing our beautiful creations can sometimes be a challenge. I'm certainly no expert on postage. One thing I do know is that a card that won't fit through a 1/4" mail slot takes extra postage. So why not make yourself a fake post slot to test out your thicker envelopes before mailing.
I cut this using the Cricut, but you could cut it with a craft knife. Use a heavy weight paper so the slot doesn't give as you slide a card through it. My sample is cut out of watercolor paper. Also I remembered to label this one. I'd cut one of these before but it got thrown away as a "scrap." I may ink the edges in some bright color this time :-)
Remember, there are many other criteria for additional postage, such as square envelopes. When in doubt, add extra postage or take to the post office to be sure your card shares your joy!