I gather feedback just after the customers finish the checkout, and the most common issue they have with FPD is that they want to confirm the design just before checking out. At first I didn't quite understand what they wanted, but after some following up I found out that they want something like what's implemented on big websites with customized products.
Let's take vistaprint.com for example, as soon as you finish the customization, it asks the user to approve it attached you'll find the screenshot, you should try vistaprint.com and customize and add a product to get an idea, is super simple and fast.
Implementing a similar solution would be great, since is, by far, the feature that our customers ask the most for FPD
This can be achieved with other plugins. This one allows to add additional fields to the checkout form: