Weddingbee boards

I am planning to print my invitations from Catprint.com. I am planning to print a 5x7 invitation on white paper, and then put a heavy kraft paper backing onto it. I'll also have a map insert and RSVP card in the white paper, with kraft envelopes for the RSVP card and the whole invitation package.

I don't know how heavy to go with the invitation and the backing. I would like it to have some weight, and I would like there to be a strong edge from the invitation to the backing. Should I be using 100lb (or heavier) paper for the invitation, or will that be too thick?

Catprint has a lot of paper options - matte card stock (80lb), linen card stock (110 lb), cotton card stock (118 lb), matte card stock (110 lb), ultra card stock (130 lb). Which should I choose? And do I use the same paper for the invitation, RSVP card, and map insert?

How heavy should the backing paper be? I'm having a hard time finding kraft cardstock much heaver than 65lb or 80lb, but it doesn't seem like that will be thick enough for the backing.

If anyone knows more about this stuff than I do, please help! :-)