I imagine any cart can be set up with a zero dollar amount. But I'm not sure you even need a cart. If they will only be using your site to select their invitation design, wording, quantity, etc then I'm thinking you could use a form that emails you the information (or saves it to a database).
I can help you design something, if you want. PM me & we can discuss...
I can help you design something, if you want. PM me & we can discuss...