Test before ordering 200
Start with 10-25 pieces, sell them, then adjust design and price. 3D printing suits exactly that kind of step.
Plan capacity
A run takes machine time. Agree a schedule so the launch does not hinge on one week.
Packaging and shipping
Design for weight and volume. A part that packs flat costs less to ship.
Quality
Agree a checkpoint: what is acceptable and what is scrap? That makes both sides comfortable.