Wonderful news! congratulations to all!
And yes, technology is great for communication! Which is why you don't need to worry about notifying the people you already mailed cards to b/c you'll be able to email them or message them in some other way once you're over there and settled in. I realize you'll be busy, but no worries, it's not your responsibility to inform everyone. You need to focus on helping out the new parents.
In fact, they might rather make the announcement to everybody, themselves, via FaceBook or whatever. So maybe you need to wait and find that out, anyhow.
Safe travels - and many joys w/ DS and family!