So sorry to hear that, I spoke with Shapeways yesterday. The printing process has a accuracy tolerance which varies across parts. For example, in brass and other casted metals it's typically +/- 0.15mm (greater or less than).
This tolerance is taken into consideration in the design of the case, however the problem arises when it skews dramatically towards to the "plus 0.15mm" side. This creates the problem you're describing.
If I adjust the design I would have the same problem when it skews towards the "minus 0.15mm" case. Hence, the problem in printing.
I have a revision of this design available in the shop for purchase right now, it incorporates a clip. However, it's currently available only in semi-precious metals. I am revising this design to work with plastic materials and will be available soon.
Again, I'm sorry for the inconvenience. I would describe this as a problem of 3D Design vs. 3D printing. Shapeways uses phenomenal technology but the tolerance required for this design is too narrow for the printing resolutions available in pretty / cheap plastics.
If you would like, I can message you when the new design is released in plastics.