How do they do it?

Just got some more parts in the mail:

10 nRF24L01+ boards for a whopping $8.12 and free shipping. Seriously, how do they do it and make a profit? I wouldn't be able to send an empty envelope to China for that kind of money...
A couple of years ago I ordered 4 of those boards for $10.40, that's $2.60 each, and now they cost $0.81...