Hello, I'm selling original paintings and prints of those paintings and I'm trying to figure out the postage. For the paintings I will be self fulfilling and prints are handled through Printful. I don't understand how to set up my shopping cart to split shipping if someone buys an original and a print at the same time. If someone buys an original painting I won't know how much it will cost until I have it packaged up and I take it down to UPS. Has anyone else set up a store like this? How did you do it?