there are no percents, you add your markup, the site does the rest. as far as i know, the prints look good. however - your work also has to look good. if its not they won't print it. so if you have images that are less than stellar - glare, blocks, bad crop (can see the edges), and so on, they may not print it and will make you fix it or you lose the sale. its a pain, but its a part of the quality process. a scanner works the best for paintings. i saw a few issues on yours, but for the most part i think they should all print. the main advantage to a scanner is that you can scan it larger and sell larger pieces (especially the duvet covers, yours don't quite fill them).
---Mike Savad
MikeSavad.com