Charging ($20) for GPL source?

[MI]

Hello,

Synology distributes it's firmware through downloads from it's website (see 
http://synology.com/enu/support/download.php)

But the GPL source cannot be downloaded. It can only be ordered on CD for a 
fee of $20. (see http://synology.com/enu/gpl/index.php).

Is that OK, by the GPL? As I understood it, if the binary is offered for 
download, the source should be as well.

Thanks,

MI