Hi Bill,
I think you could use different photos for the listings. Like for canvas wrap, the photo could be taken at an angle so that you can see the side of the canvas and know that it is a canvas print.
I sell both digital and printed cards, so I have to do double listings. But for digital listings, I have a banner on the first photo, that says that this is a digital listing. So it differentiates the listings a bit. You could do that too.
If it's too tedious, just use the drop down function in just a single listing. I think it's perfectly fine too.
I hope that helps.
-Khin