I happen to have purchased various chilis, including the Black Urfa and the Smoked Pimenton, from Burlap and Barrel a few weeks back. Both are delicious, and, I felt, a decent value given the quality. I don't have any experience with the sumac.
(I typically buy my spices from Penzy's, so that's my reference.)
kaxixiSumac adds a solid lemony tang, so its great for hummus, lentil soups... anywhere you'd add fresh lemon to make the flavor more complex.
Its also a nice deep purple color to dash as a garnish on top of a dish.
Oh and price here seems alright, given its in a glass jar.