Custom Orthotics import

We are starting off a custom orthotics resale business. We supply the podiatrists with a diagnostic machine. The results are sent to the lab in Europe online, they produce custom orthotics there and ship them to us in the US. The question is what document should we get in order to be legally importing custom orthotics for resale here and not get in trouble at the customs, when they would ship us 100+ pairs. And what kind of supply contract shell we sign with the lab in Europe. Thank you so much for the detailed answer.