I just today mailed several (about 10) of the invitations to my son's birthday party. I used padded envelopes from Office Depot that were 12 for $5.50, which wasn't too bad for padded.
I didn't have a gel card, but I did have a card that included two regular crayons, and the total weight was less than 2 oz., so the postage was $.60 each. Not too much more than the $.37, so I went ahead and did it.
However, different people have different definitions of "too expensive". Your Mileage May Vary!
Penny