How do I add an interstitial page/view after adding a product to the cart?

We currently do not allow for you to easily inject a new view into your customer's workflow.  This is an intentional decision, to help mitigate the negative impact such a thing will have on cart abandonment.  However, it is indeed possible.  Here's a generic list of steps to make this happen.

1. Add and use the custom viewset extension point on your site
2. Rename the Add to Cart button to something more meaningful on your product details view
3. Add a querystring parameter or session variable to the Add to Cart button
4. In the top of your cart view, look for the querystring or session variable, and redirect to your new view if it exists
5. Perform any logic desired on your new page, and redirect to the cart once you're done

Exactly "how" you save data to any data source is completely up to you.  All standard methods that are used in the CMS and ASP.NET in general are available to you.  

I'd like to point you to an alternative approach though.  This works quite well and it doesn't appear to have a major impact on cart abandonment so far.  Overall, it's easier to manage and better integrated.

1. Add and use the custom viewset extension point on your site
2. In a new and renamed view for this and other products, perform the steps below
3. Don't rename the Add to Cart button, but replace it with a link that still says "Add to Cart" 
4. Add a (mobile-friendly) popup up that occurs upon clicking the new Add to Cart button you just added
5. Include any form fields and other information into the pop-up 
6. Add the Add to Cart button into the popup, using the same markup from the original view
7. Add and use the action delegate pipeline extension point, specifically the BeforeProductAddedToCart event to retrieve and save the form field information from the pop-up
8. Follow the examples in the previous step to send the customer to the cart once you're done