But come on... is it really that big of a deal? Like victor_jenkins said, the client just paid and didn't complain. Maybe the price was actually fair for the work done. New people always worry they're charging too much, but sometimes the client thinks it's worth it. We don't even know what the service was. Making a mistake on your first rate doesn't mean you ruined someone's life or something. It's just part of figuring things out.
Actually, I get what Tara's saying, but trust needs a starting point. When you're new, you have zero experience to base that judgment on. It's easy to second-guess if your price is fair or way off. That confidence to set rates comes much later, after you've seen how your work lands with a few clients.