I received a letter from Johnson and Pham LLP, what should i do?

On July 10th I received a certified letter from the US customs and border protection regarding of seizure notification for counterfeit merchandising. Apparently, there were 52units of fitness products of Focus T25. However i didn't order anything whatsoever of that nature. by the way, They held the package and was never signed tho. But it has my full name and address. So I called the custom penalty fees department for advice and was told to do a handwriting petition letter explaining what happened then send it back along with a check mark option stating to abandon the property, so i did. But few days later i received a regular first class letter from the law firm "Johnson and Pham llp" attorneys basically demanding me for trademark, copyright infringement, false designation of origin, trademark dilution and unfair business practices. Also want me to send them a check for $15000 for the complaint damages. What are my option at this point? Thank