We are working right now on adding ability to integrate surveys into any page.
So this way you can integrate into checkout process and any other pages of your store.
Will also add few premade styles for inline/condensed surveys.
Existing customers will be able to download this update free of charge.
From business point of view I'd not recommend to do this - ideally you shoudn't disturb customer during checkout, probably on checkout success/failure page only.
Go go even more - I'd recommend to use our both products together -
Follow-up Autoresponder +
Survey Builder to get constant feedback from your customers based on multiple criteria:
1. How is his shopping experience - good for checkout success/failure
2. Is he happy with product delivery time
3. How he likes the product itself.
4. What else he would like to buy from you
etc
And you can have all of this automated!