X-Cart plugin is very specific, it does not integrate checkout process (as many would expect) so we sell it only by request. Contact us via helpdesk for payment link.
aMember will just add users to X-Cart users database when they are registered in aMember, and do "single-login" between X-Cart and aMember. NOTHING more. It works as described here: http://www.amember.com/p/Integration/ I will send you payment link via PM.