Off topic: Where can I buy cheap certificate?
You just have to be able to receive email to webmaster@yourdomain.tld for that to work.
GoDaddy is the cheapest, but I hate them. Use Namecheap.
> Class 1 certificates are limited to client and server certificates, whereas the later is restricted in its usage for non-commercial purpose only. Subscribers MUST upgrade to Class 2 or higher level for any domain and site of commercial nature
Class 1 is "StartSSL Free", and Class 2 is "StartSSL Verified" which costs $59.90.
[1] http://www.namecheap.com/ssl-certificates/comodo/positivessl...