From what we've been seeing here, in bits and pieces, the royal invitations are made by a special engraving process that is at least a couple of centuries old. I expect it's expensive enough just to have best quality card stock. Adding furbeloes to those traditional invitations just doesn't seem right - but I keep hearing that Kate doesn't want a terribly extravagant wedding (!)
I was wondering how much professional calligraphers might charge in the U.K. to address all those invitations - it's quite expensive here. The personalized touch of writing in each invitee's name is well worth it, I think, but I doubt that those companies who cater to we common folk would find it hard to allow for such an option (the paper has to be just right - it cannot be to thin and porous).
Myself, I would have been ecstatic with the invitations that they chose.