Reach out to them and find out the correct facts and then react accordingly ! If they are correct in their feedback , accept the fault , thank them for pointing it out , mention that it is a very rare case of a mistake having happened from your end, and offer profuse apologies in the reply to them (on the site ) ! If they are wrong ,say that you reached out and found that the issue or the understanding was different and point out why ! But don’t get into a verbal fight on the site —you may win the argument but also lose hundreds of customers !