I finished reading that tutorial and was just about to ask the same question:
Do you really need to buy a new certificate every time you want to add a new domain to the multi-domain certificate?
I am asking because they actually make you pay for the verification process, the certificates are free but you need to verify your identity for the multi domain certs.
I'll ask them too if it would be possible to ask for a new free certificate every time I add a domain to my hosting portfolio or not.
If needed I'll get a certificate for my hxxxxx.stratoserver.net and have them all use that for accessing their emails.