Martin, Open ssl and commercial ssl certificates have some differences. First of all, no guarantee is provided when you use a free ssl certificate. But if you buy a commercial certificate and it is not working for some reason... well, you have a place where you can get it working
and may be get a refund even. Usually when you pay for something , the party which has the responsibility for that product has to do everything to provide you with the quality product and the guarantees, etc.
I hope you understand what I mean.
Lehrer