How to address wedding invitations.... I repin and laugh. Why one earth are two envelopes necessary: ever? For anything? It all gets thrown out and forgotten the day of the event. Pure waste. No one cares how many envelopes you send them.
The formality of your invites will depend largely on the formality of your event. For kicks, let's say this is a traditional Victorian tea party: in which case, you'll elect to send a handwritten note on a tea card, which is a notecard specially embossed with images of English roses, tea sets, and other lady-like paraphernalia.