Google Sketchup: Fill unseen space?

I'm trying to get a cheap 3D print of a model(<$100 with S&H). But one problem is that the walls are infinitely thin, so I need to fill the space between them. But how do I do this? The main problem is the boxes all around the surface(look at the uploaded model). They need to be filled, without the internal space where the flashdrive goes being filled. Can someone please tell me how to do this, and also a cheap 3D printing service?  Cheapest I have seen was i.materialise. The price for the model: $36. Not bad at all. The price of shipping: $144. I'm not going to spend almost $200 on a case for a flashdrive.

You just need to export a DAE file.

Here's Ponoko's instructions:
metrogdor22 (author)  fungus amungus6 years ago
The problem with that is I still get a "non-manifold mesh", the most likely culprit being "zero thickness geometry".