my first faux pas

I apparently committed an etiquette faux pas. I called my mom (yes of course this story includes my mom) today and she says, "Aunt So and So got your save the date today. She didn't really know what it was. But she told me you didn't include their last name on the envelope."

That was true. I didn't. Somewhere on the fine internet I saw a save the date envelope with first names only and thought, oh cute, a nice informal way to do these. Well my mom is now appalled that I don't know how to address envelopes and that I will surely offend all of our guests. Um? Seriously?

First, I don't get how this is an etiquette thing. I understand the logical reason to include last names: to insure that the postal service delivers the envelope, especially if I mess up the address. However, if the envelope did get lost, no one could possibly get offended because they'd never receive the envelope to offend them in the first place. Secondly, if I have a personal relationship with someone, I kind of don't get how using a first name on an envelope is offensive. For people I don't have a relationship with, I used the last name because I was uncomfortable - we're not on a first name kind of basis, you know?

So my mom is pissed at me because I didn't use last names. Have you committed any grievous etiquette sins?