Custom forms/survey at checkout for specific products

[Hilary10]

For some of the subscription products I sell I need to get specific information from the customer in order to tailor their subscription to them.  So I am looking for an app that will allow me to make a custom form that either pops up when a customer adds a specific item to their cart, or is within the checkout when a customer buys a particular item.  I need a different form for different products.  

I've looked at a couple of apps and only found either post-purchase surveys, or forms that are embedded in the actual product page.  Neither of those suits my purpose as I need the form to be filled in after the customer adds the product to their cart but before they complete the purchase.  Does anyone know of an app that will do this?  

[nack]

I have a similar need. I'm moving over from Squarespace while Shopify blows away SQSP with features one thing that it has that we really like is "Custom Forms." Which looks like: 

[Monicalsl]

Did you ever figure out a fix for this?

[37Hertz]

I still need solution for this.