Andrews Communications here in Oz has the GSO 2X barlow for $29. Add $10 express post to anywhere in Oz. The Barlow section is a little past half way down this page:
http://andrewscom.com.au/site-conten...-guansheng.htm
And here is the scope's home page of Andrews, as the GSO page doesn't have a 'home' link:
http://andrewscom.com.au/site-section-10.htm
All barlows are achromatic, unless they expressly state 'Apochromatic' which is uses superior lens materials and lens arrangement.
This GSO barlow is very good. Would not be surprised if the Seben one comes from the same place.