One idea for the invitation is to run the letterpress design with no ink, so the pattern will just be imprinted deeply in the paper with no color. Here is an example.
I did a quick mock-up of this last night using a dover peony clip art I found and oak leaves I had from a previous design. The peony would be imprinted lines only and the oak leaves would be a full stamp. It's not perfect, but it is a pretty good start. Click on the photo to enlarge if you'd like.
The challenge is how to incorporate this all ivory design to match the yellow stage design I'm working on for the RSVP and save the date notes. Yellow envelope? Yellow ribbon tied around the invitation? Yellow border around the frame of the invite?