The final issue with invitations and etiquette concerns guests who do not RSVP. Do you assume they are not coming? Do you decide your invitation was lost in the mail and resend it?
The quickest and best way to resolve this is to call all non-replies over the phone and confirm their RSVP. Family members can assist with this. The only persons who don’t need to reply are your parents and future in-laws – in fact, they do not require an invitation at all but can be sent one as a keepsake.