Thanks for this. Hadn't actually considered a quarterly charge. That's a good idea. Low monthly charges start to get difficult with per transaction standard fees (like paypal).
One more quick question. Are you acting as an individual i.e. freelancer, or agency? I don't think an agency would mind paying $12/month if they had a few hundred domains, so perhaps the domain limits are off and it would be better like this:
Professional - $5/month - 100 domains
Agency - $12/month - 500 domains
Max - $30/month - 2000 domains (can't see much demand for this plan)
I agree the free plan doesn't seem to make much sense. A better option would be to give a free trial period on the paid plans e.g. 7 days.
