Unless that second letter is legally binding, and notarized by an attorney, etc. you do not have to respond at all.
Legally binding means that not only does the letter state what you need to do in correspondence with the law (e.g. your reply to the seizure of goods), but it also needs to state in legal terms the legal consequences if you do not respond (e.g. references to certain sections of laws of your state, etc. such as "if the party does not respond, they are hereby punishable under the full extent of the law to "such and such" in accordance with "section ## of ## of the "Act/Law/Jurisdiction passed in said year.")
So what I mean is you're technically not obligated to respond unless the law itself threatens you to do so, and I'm assuming that it is just customs itself requesting your admittance/ownership of the said goods (as remember that you can claim in a court that someone used your name and address without your knowing, etc.) Unless it is like an actual representative of a copyrights protection company, or legal section of a company, then yeah don't respond.
^_^
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