The Wedding Announcement

I got a wedding announcement in the mail. And seeing as I never took any manners courses in school (were they even offered?), I have no idea what I'm supposed to do with this thing. If they want money they should have included a self-addressed-stamped-envelope that I could have stuffed with some cash. This has been a recent theme in my life: someone does something that should trigger some kind of etiquette reaction from me, but I have no idea what the norms of society dictate. And when I found out what I am supposed to do, it always sounds hopelessly antiquated to me. So I guess I'll look online and see if I now owe these people who didn't invite me to their wedding money. I'm guessing I do. That's just how weddings are.