I agree that per item shipping settings would be awesome, and someone up in the thread kind of described how to do it, and it was really difficult for me to wrap my head around it. This article from the Printful blog was extremely helpful.
https://www.printful.com/blog/match-printful-shipping-rates/
I went ahead and set up that same scheme for my Printful items. The challenge I've got is incorporating items not fulfilled by Printful into that same shipping rate scheme. Anyway, I hope this helps!