What kind of paper should be used? (invitation envelopes)

Would regular copy paper (regular everyday printer paper) be too thin? We'll have both inner and outer.

Suggestions? Thanks.

here's a site I found on DIY envelopes.  They suggest cardstock... I would use the lightest to save of postage costs!

http://www.do-it-yourself-invitations.com/wedding-invitation-envelopes.html

Thanks for the link!

Thank you for the link!