Individual SSL Certificates on Mapped Domains on Multisite?

Hi, I'm trying to setup the Stripe payment gateway for one of my clients. It requires an individual SSL (specific to the domain) in order to work. I've been advised that just a simple one like those Namecheap sells for under $10/yr will work just fine: http://www.namecheap.com/ssl-certificates.aspx

However, I have no idea how to go about setting this up for a Multisite Mapped Domain (using WPMU DEV Domain Mapping plugin). I've only ever setup WP with SSL once before, and the hosting provider setup the certificate. So I'm just trying to get it figured out before I proceed.

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On a related issue, I read that if you put "https" in for the WordPress Address (URL) and Site Address (URL), it would force everything to run through https/SSL. On this other site I mentioned (single-site, not multi) I believe it was working that way before. However, now when you visit the http version it stays http, even though it's set to https as described. Any ideas on how to always make it go to https?