Some things to consider on shapeways:
They have flat shipping fee
Each material has a handling fee, so order all your things in the same material if you can
The reason to use shapeways is the shop feature. Others ship much faster than shapeways. I've been using other printers for my items, so I have fast turnaround for printouts that I then need to change. If I waited for shapeways to print, then make the changes etc then reprinted, changed reprinted etc I'd be looking into making a shapeways shop in 2015( not literally, but you know..)
For casting, use a different material for the prototype. No problem to cast in several of their materials, right up to silver, gold plated, etc. choose a good material that is non porous and no problems.
For scale items: do not make 1/18 for gi joe as it is wrong. I have spent AGES on this and 1/16 is the right scale for gi joe items. 1/18 might be someone's preference but it looks very small and the difference is noticeable for sure.
Scale guns need to be thicker than is correct for scale. A correct thickness gun is incredibly thin ( looks about 20 percent or less of the thickness of an mj gun).
If you want to see the correct scale, send me your email and I'll send you a pic of a mj Ak next to a 1/16 scale, and they are the same scale and look right.
For shapeways shops: order your items n the same material, as you pay less per item due to handling fee. Shipping is a flat rate too. For small items put them on a sprue or they loose them.
If you have any questions, let me know...I use shapeways but am doing a lot to make it optimised for what we need in our scale. There are a lot of potential pitfalls with their service...