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You can't use the Free StartSSL certificate for commercial websites.

I can't seem to find that info on their page. Do you have a link?

Not really.

The more expensive certificates offer insurance, but if you read the small print you'll see that it isn't worth much.

Free startssl certs only run for a year. For me, it's worth paying for e.g. a 5 year one to put off the renewal faff.