After customer cleans his browser history, there modifed design is gone...
That's not good
The best solution is to require the user to login or create an account before they can customize the product. However, this would be a function of your shopping cart not PitchPrint and likely will require a bit of programming knowledge or a developer to implement this functionality.
Am sorry if they have logged in before creating their designs, then the design is saved in "My Accounts"
If not, they will loose the design because they cannot be identified but are mere guests to the website.
I don't see this as a problem. This is not a PitchPrint first, if you create wishlists on OpenCart or any other framework without a way to identify you, you loose that data if you go the extent of "Settings -> Menu -> Privacy -> Clear Cache and Saved Sessions" in your browser.
If you log unto FaceBook or any other site for that matter and went to the length of clearing your cookies and saved sessions, then you loose obviously all sessions.
We can't force people to login or register before allowing people to create designs, this puts a lot of customers off in the first place.
So am going to set this as a solved issue, it being not an issue at all.
Maybe the future ability for the customer to log in and save their designs that way?