To expand on what mkroeker said. . .
I took a look at your file. Nice geometry! What software did you use to make that?
It appears that your modeled is in inch units. When we do uploads we check the model units that correspond to the units of the file since this information is not stored in the file when it comes to polygonal based file types like STL or OBJ. You can set the model units here:
As you get into 3D printing you'll notice that just about everything concerning 3D printing uses the metric system, even though it was first invented in the US. I used to do all of my modeling in imperial units, even for CNC, until I got heavily into 3D printing and then I dropped imperial units altogether and only work in the metric system now. It's still a matter of choice, but it makes things easier if you go metric when you're going to get heavily into 3D printing.
Your model is not printable in any of Shapeways' metals!
Don't worry though, we'll get you fixed up and printing in no time. Yes, the walls are too thin, but there is another problem. The cube that you have inside is shielded by the sphere on the outside. What I mean be shielded is a sprue needs to be attached to each part so that it can be filled with molten metal during the casting process. Consequently, what you would need to do is make that cube a part of the outer sphere like you did for the bail. That way the casting crew can simply attach a sprue to the outer sphere and the metal will have a way to get to the cube inside.
However, before doing that, you'll need to thicken those walls. The cube would be fine if printed in an unpolished investment cast metal since it has a minimum wall thickness of 0.635 mm. The minimum for unpolished is 0.6 mm. There are other investment cast metals that Shapeways offers that do not come with the option of unpolished like platinum, gold and the plated metals. So, if you'd like to have the option of printing in all of the investment cast metals you'll need to increase your walls to at least 0.8 mm.
There are two other metals that Shapeways offers and those are steel and aluminum. Both of these are done differently from 3D printing a wax pattern and then investment casting and have much thicker wall requirements as consequences of their respective processes.
But wait! You probably want that cube to be free to move within the sphere! Ok, that is possible in most of Shapeways plastics, in fact it's routinely done in plastic and is one of the great benefits of 3D printing in plastic. For metals however, is gets very difficult. All of the metal processes need a sprue attached to get the metal into the part by either filling a mold in the case of investment casting or drawing it in via capillary action in the case of Shapeways' steel. Shapeways aluminum melts a metal powder in place and therefore doesn't need sprues, but the problem here is it needs support structures to keep the metal from warping while printing and so that won't work because there are no easy ways of removing the support structures that need to be inside the sphere.
There is hope though. Take a look at
this. This is Shapeways' experimental interlocking metal pilot that you need to signup for in order to try it out. It uses investment casting and has the same wall thickness parameters as Shapeways regular investment casting, but you can have up to six separate interlocking parts per order. The thing is though they need to be able to connect one or more sprues to your inner cube. Once you get approved for the pilot you can get your geometry corrected and then see if Shapeways can do it. Shapeways always has a human check your geometry before anything is printed so they will tell you if something is not going to work. The problem would be sticking one or more sprues through your star pattern holes, casting it, AND THEN ALSO, removing the sprues after casting. If Shapeways says they can't do it you might try making the spherical star pattern less dense so that the cube sprues could be worked with.
Oh yeah, one more problem you may run into with this design. In jewelry design we call this a filigree design. The problem with casting filigree designs is getting the molten metal to flow everywhere to fill up the entire filigree pattern. To make it work lots of sprues are attached all over the filigree to try to let the metal flow in as freely as possible before it solidifies. If not enough sprues are attached the filigree will only partially fill and the result is a casting failure. Thing is though, Shapeways doesn't want to spend all day attaching a bunch of sprues that then need to also be removed after casting. So what you'll get from Shapeways is a rejection notice entitled weak geometry. We all get these various rejections all the time. So, no need to get stressed over them. I remember when I was in your starting out position years ago I received numerous rejections on my first design that I worked on. I wanted to strangle a few Shapeways workers at the time! HAHA!
Nowadays I know that each of those rejection were justified. What you would do if you did receive this rejection is think about how you can make your geometry thicker or change the pattern so that the metal can flow more readily.
Anyway, once you get the hang of it it gets pretty darn fun! Almost anything you can draw up in 3D can be sitting in your hands a week or two later and that's super fun and also SUPER ADDICTING!