This is a great idea , it all comes to privacy on the address that is shared. How are those preserved
The card is however printed by a third-party with whom we thus have to share the address details for shipping. From there on the is no additional privacy concernes compared to sending a postcard by hand.
I think you should add a statement addressing the privacy issue. Will you or won't you sell data? Is the data encrypted? Will the vendor not sell the data?
I can see this product being a lot of fun. But not at the expense of loss of privacy.