Just today, I was in a conversation about postage stamps on your Save the Date's...is this important to you? The bride from today is getting married in December, and when she went to purchase her stamps (post card stamps) the only style the post office had were Hawaiian shirts. She felt this to be ironic, a winter wedding with summer theme stamp. Ironically, another bride who was married last summer also sent out Save the Date post cards; her stamps...Polar Bears! Is this something important to you? Do you think your guests will notice the postage?
Today, post card stamps are $0.32 and the only style offered by the USPS are those Hawaiian shirts. The other option you have is to custom make your stamps online. Web sites like zazzle.com let you upload your own design or picture. They offer all stamp sizes (small, medium and large) and all rates, including the post card rate! The total price is a little higher because you are paying for the customization. But for one sheet of 20 post card stamps, you will pay $16.95 (buy three sheets and the price comes down to $16.45 each sheet). This, unfortunately does not include shipping, there was a $4.15 charge to NJ. All in all, I don't think this is too bad...to order 60 post card stamps (3 sheets) including shipping; I would be paying $53.54.
Fortunately, the United States Post Office does offer a nice wedding selection for your invitations. I would recommend using their stamps for them as you will already be paying for extra postage due to the weight of the "packets" you are sending out.