No problem for me when I test the link. To be more specific, it is offered by Electronics Basket (and Ritz for slightly more) - both fulfilled by Amazon.
Try this version of the link:
http://www.amazon.com/Pentax-DA-40mm-2-8-Lens/dp/B0078KOEMA/ref=sr_1_1?s=pho...Pentax+40mm+xs
As for comparison of the 35 f/2.4 and the 40mm xs, it really depends on your needs. I'm not fond of 40mm particularly as a normal lens (60mm equiv.), but the size is really attractive for stealth purposes. I can imagine bringing the 40mm along with the DA 21mm, and have quite the range of choice with one lens staying in a small pocket.
I have a FA35mm f/2, as well as various 50mm (old MF and current f/1.8), so the focal length isn't the main attraction. For me, I'd say this has mostly to do with cheap LBA.