@Ebony_Aisha: “My thoughts” are, that traditional proper etiquette is largely on your side, and would encourage you to cut back your paper use even farther:
Traditional etiquette’s close cousin, traditional aesthetics holds the principle that “Less is more.” The most elegant invitation is a single card without additional inserts and certainly without unnecessary ones. It is properly your guests’ responsibility to reply correctly: in writing to a written invitation, verbally to a spoken invitation and by extension, by telephone to a telephoned invitation, and so on. Socially sophisticated guests are supposed to know this, and to own their own stationery and stamps for taking care of the “in writing to a written invitation” part, so no bride has any obligation to provide R.s.v.p. stationery and stamps for her guests. She just needs to provide them with the appropriate address. Traditionally this is done by putting the letters “R.s.v.p.” in small print in the lower left of the invitation, followed by the address (which in your case would be a web address). As long as your website has nice clear instructions on its home page, that is all you need to put. No instructions on the invitation. No extra card insert.
Of course, your socially sophisticated guests who know traditional etiquette and are sticks-in-the-mud about it, will still take out their personal stationery and write:
Miss Aspasia Phipps
accepts with pleasure the kind invitation of
to her wedding on Saturday the ninth of August
and would prefer the Griddled Tofu Cordon Bleu
so you won’t be able to save the environment from the gratuitous paper-use of folks like us. But most of your guests will follow your hint.
Now, if you want to save even more paper, you can forego inner envelopes as well, as long as you word the invitation proper so that it has a space where you can write in the names of whom you are actually inviting. The traditional wording would be along the lines of this:
Miss Ebony Aisha
requests the pleasure of the company of
to her wedding on Saturday the ninth of August at eleven o’clock in the morning
The “Miss Phipps” is hand-written into the space left on the invitation. Or, in this lovely modern age of computer-driven print runs, assuming you are not having traditional copper-plate engraving done, you can even do a mail-merge so that your invitations have the “write-in” line pre-filled in the same font and ink as the rest of the invitation.