I'm just worried about submitting all that info through some web form to some dude.... it'd be nice if he just set himself up as an Ebay service. That would be really nice, in fact.
The cart takes you to the ecommerce part of sqarespace.com as https:, so I'd say its secure. If you have an issue ordering thru him google squarespace and see if anyone has issues with their ecommerce site.
Personally I like to support this guy as he put this bracket together long before NZXT.