Except for SSD's, perhaps its just easier to use something like http://www.dban.org/ and DIY. If you're a little paranoid, it isn't hard to imagine the USPS waylaying mail hard drives en-route to Destroyer.io for forensic imaging before it reaches its final resting place.
I understand your concerns, but this particular scenario seems a little far fetched. USPS would have to have a very strong interest in your hard drive. Know when you ship it. Intersect the package. Repackage it up exactly as it was. And finally, deliver it. This is extremely complicated, and they would be breaking an immense amount of laws. If they had such a strong interest in getting a hold of your drive, they'd be…
- http://www.engadget.com/2014/05/16/nsa-bugged-cisco-routers/ - http://www.theguardian.com/books/2014/may/12/glenn-greenwald... - http://www.theverge.com/2013/12/29/5253226/nsa-cia-fbi-lapto... - etc.
At the very least, shipments should be made in a strongly tamper-evident container.