How to Accept and Regret Formal Invitations

Ever wondered why some invitations—usually the ultra fancy wedding ones—don’t include RSVP cards? Believe it or not, these convenient cards weren’t always included (and for formal occasions, still aren’t) because etiquette dictates that the invitee send his or her response on personal stationery.
Acceptance

Formal invitations should be accepted or regretted in the third person
They should always [...]