Would like to create parametric models for users to customise. What's the best way to achieve this?

I'm just getting started designing a few things, but I have programming and 3D experience and training. I'm probably going to start with a few fixed designs to get my hand in, but I'd like to eventually be able to offer customisable designs, so users can tweak parameters and get personalised results. I know that Shapeways has an API, is it capable of this kind of thing, or is it more of a shopfront API?

ShapeJS could be what you are looking for.

That is definitely what I'm looking for. Thanks very much!