Mechanical Design Engineer: 4-color retractable pen mechanism (3D-printed prototype)

I’m developing a larger-than-normal version of the classic BIC 4-colour pen and need a functional prototype for further development. The job is to take my detailed specs (spring rates, hole sizes, tolerance notes—shared after NDA) and turn them into a working mechanism that reliably locks and releases each colour.

Scope
• Model the full mechanism plus an outer body that lets me swap refills without dismantling the whole unit.
• Produce STEP and STL files.
• 3D-print the parts, assemble, and fine-tune until every push button clicks in, stays locked, and pops back smoothly.
• Document the build with clear photos and a short video showing all four colours engaging and disengaging.
• Optionally ship the prototype to me—let me know cost and carrier options.

What I’d like to know from you
1. Examples of similar spring or push-button mechanisms you’ve designed.
2. Your in-house 3D-printer type and the materials you’d use for both housing and moving parts.
3. Separate pricing for design and for printing/testing, plus estimated turnaround.
4. Shipping availability and cost to my location (details on request).

The emphasis during testing is on rock-solid lock/release action for each colour; comfort and refill access are secondary but still important. If you enjoy precision mechanical design and hands-on prototyping, I’d love to hear how you’d tackle this challenge.