I was playing around with some of the prices on my posters at Zazzle, and I noticed that you can sometimes UP your earnings without changing your price. Like I had one poster selling for $14.75 at 10%--and I was making $1.48 off of it. When I changed it to 11% it still showed it at $14.75 but it showed my profit as $1.62. I don't think this has to do with having to wait until the 20th of the month to see the changes, because this was the preview of what the price would be. I will check back after the 20th and see if the $14.75 price holds though. (I checked...they did).
Not going to make a big difference or be worth your time if you sell small amounts, but if you sell a lot of a certain item this could add up.