Incorrect postage on invites

Hey yall! I’m having so much stress over this. I put together all of our invites (wax seal on vellum) and brought one to the post office a few weeks ago to ask about postage. The woman weighed it quickly and said “it’s 1.4 oz, so you can do a 2oz stamp to be safe and for ease or a couple different kinds.” So, we just did a 2oz. We got one returned back today and it said insufficient postage. I sent one to my mom as a test earlier in the week and it arrived fine. Now I’m paranoid they’ll ask our addressees for the additional postage difference (which Google says can happen)! Has this happened to anyone?! HELP