Given the cloud ecosystem (and associated markets), I would be noteworthily surprised if an average datacenter/colo's smart-hands facilities wouldn't be able to accept me mailing in some HDDs, having them attached (possibly by renting multiple additional servers and having them installed in there), then later disconnecting them and printing a supplied prepaid shipping label to return them.

(I'd provide an appropriately oversized tip to ensure a near-unreasonable amount of bubble wrap is included in the return.)

I might be reading this wrong (I always have problems calculating AWS costs), but this seems VERY expensive. Much more than paying a 10Gbps internet plan for a year where I live.

Possibly, but is cost the only factor? What if you don't want to wait a year to get the data? Or don't want to saturate the 'net connection you're using with just this download? And what about error recovery, since no 'net link is going to be 100% stable for an entire year...

Not saying Snowball is the best idea by any means. But I can see how it might be worth considering.