I worry about your business model though. It seems nice to be able to safe a thousand or more dollars on wedding invitations BUT here are the problems:
1) When a wedding costs 25k, saving 50-500 bucks on an invitation is going to be looked at as stupid, and cheap. You want the one that takes the most hassle out of it.
2) A big part of the invitations is the printing/mailing. If only the design was the only factor.
3) Often catering halls (im from new york) will do everything from getting the place, getting the catering, getting the flowers, getting the invitations and sending them out, its a full-blown package.
The real way to go about this is to have people design their invitations in an awesome way quickly before their wedding, get hooked to it, and already have that in mind. Makes it less of a headache for them, and sells your product.