This is the hitch supplied with the trailer when you order the SWIVEL OPTION. The cost is $37 (US $30) in addition to the price of the trailer.
This hitch itself is available as a part for people building their own trailer. In this case the price of the hitch is $94 (US $74), plus packaging and shipping.
An alternate hitch is the Holistic Hitch (click here for pic and prices).
Both hitches require you to attach them to your trailer towbar, so welding is usually involved.
And of course they attach to the seat-post-the best location so you get a handcart as well as a trailer.