Thanks for the response! I should clarify that I'm not a new calligrapher. I'm confident in my lettering--just not in envelope paper. I have to order LOTS of envelopes, and I need to make sure that the ink won't bleed or feather on the envelopes I order, or wash away in bad weather (your comment about that was very reassuring). I've found suppliers don't list (or even sometimes seem to know) the content/finish/weight of envelopes as reliably as they do for sheet paper, and it's making me nervous about ordering.
A fellow guildswoman gave me some samples of things she's used just for personal pretty mail, so that's helping--I don't LOVE the envelopes' feel but they take ink beautifully, so if I don't find something more substantial and luxurious-feeling, at least I have an option. I've ordered some samples, as well. The kids aren't in as big a rush as they feel they are, but they have so many things to be nervous about that they are nervous about everything.
And the bride's family are being a bit aggressive in the planning, so the engaged couple is trying to do as much on their own as they can while going to school and working, which makes it harder to reassure them that things will work out fine.