I just got a wedding invitation in the mail from people I do not know. In Kentucky. It doesn't seem to be a mistake, as my name and address are handwritten on the envelope. I don't know where they would have gotten my name/address. I don't even know if I want to mail them back saying "who are you people" with the small possibility that I have just forgotten their names.
Friends of a family member, perhaps? Or friends of a mutual friend hyping for "send a gift even if you can't attend"?
Show up for the wedding, eat all the food, abuse the open bar, and jerk off onto the wedding cake. If they don't know you and you don't know them, what's the harm?