The obvious and straight forward answer is that we don't do that. The more complicated one would be that we'd break a large amount of serious laws if we were doing something like that.
When testing the service out, some users asked for further assurance . The solution that seemed to be liked the most involved a video feed (or pictures), that showed how we opened the package, removed the drive and destroyed it. That's something that I'd build down the road if I get enough requests or feel that it would move the needle in the right direction.
For now, here's what we do: receive the drive, degauss (demagnetize) it, destroy it. The customer is notified when the package arrives and when the drive is destroyed, at which point we send a certificate of destruction.
Also, we're in the process of getting our NAID AAA, eStewards, R2 and ISO 14001 certifications. The machines and processes that we use follow all of their guidelines, it's just a matter of getting approved (and paying the fees).
Let me know if you have any other ideas on how we could show/proof that we're trustworthy. Interested in listening to any suggestions.