Moving to InDesign EPUB forum
That is way too complex for an EPUB. I would think you’d need some kind of website with a backend designed specifically for this.
I hope you’ve got a large budget.
If you are happy to crack open the ePub and you don't mind it not validating, you could create an email from your ePub.
Your postcard interface could be a text area for the message. A MSO to choose a nice picture for the card. A text area for the email address. A submit button would bring the bits together and launch the email application.
Here are a couple of handy resources for creating emails.
Thank you for your reply!
I will look into this and get back to you on how i get on.
I have an ePub with a working submit button, if want to take a look a some possible code.
You are a master at pushing the envelope.