Understanding what Pic Rights is will help you. They are not the copyright holder and they are not a law firm. So, they cannot sue you in any capacity. That does not necessarily mean you should ignore them. They represent some very well established companies. They are some some sort of license compliance company. If they do not settle cases, they typically shop them to attorneys. I am not sure if every case that does not settle accepts accepted, but there is lots of indication that a large number of their cases get handled by attorneys in the US.
If you think you did infringe, you might want to consider paying the amount or making them an offer to accept a lesser amount. If you have economical access to an attorney, you can also start there.