IF you get the postage reimbursed, you should thank your lucky stars. I've had a dozen or so similar events over the course of my lifetime, and I haven't even ever gotten anyone to listen to my complaint, let alone respond to it. The fact that someone returned your call is a freaking miracle. Don't expect anything else.
The USPS's liability is limited to the postage, unfortunately. Hope it's not too late to scrounge up some evites or something. Congrats on you marriage, anyway. :)