15% is a pretty good royalty, especially since jewelry designs are
generally sold outright. The fact that you retain the right to
produce and sell the design yourself is a nice extra. Most contracts
would enjoin you from making and selling the piece yourself.
Really, there’s not much of a downside. Since they do all the
manufacturing and marketing, if the piece doesn’t sell on their
website you’re not out anything but your original design time.
I’d contact some of the designers they sell and ask about what their
experience has been. Does Shop Bevel seem trustworthy, etc.