Hi, your question is not quite clear. A client can only complete an order when you deliver. So I'm not sure how the client accepted the order by mistake. If you were able to deliver the order, but the client didn't look through before completing it, then yes you can deliver via the inbox. And no, this shouldn't have any impact on your gig ranking. But it's important that you reach out to the client early enough so they don't send a support message.
Also, make sure you do not deliver empty works. If you do that, you risk being penalised, when the client reports to the support team.