If you want to order personalized stamps for your invitations, here's a coupon code (think it might expire today though).
Go to
http://photos.stamps.comCoupon Code:
psgift15off2011
15% off!
Make sure you choose the right postage amount (45 cents for regular letters, 32 cents for post cards). Postage goes up 1/22/2012.
Enjoy!