So my invitations all printed up, envelopes addressed, and about half are sealed up and about ready for mailing (need to add stamp). I intended to give a +1 on all above 18 guests who were not married or in a committed relationship. This includes a few of my cousins, who's invitations are sitting in the sealed and addressed envelope pile, without being noted that they could have a +1, as that is sitting near the invitation pile for the ones that still need to be sealed. Nobody involved has to travel far (all within a 20 min drive), and the dress encouraged (if asked) is casual (something on the top, something on the bottom, preferably no holes).
Should I just contact them individually and let them know word of mouth or should I open their invitations add the note and then reseal them/ go find the right size envelopes.