Through my 10+ years of Airsoft d parts importing experience, (both personally and commercially) I have dealt with a number of seizures, and have won a fair amount of appeals.
My advise is to wait for your seizure notice, then follow the procedures given to you to initiate an appeal properly. They have no ground to seize your handguard and extension barrel. Appeal it, stick to it, and you will win in the end.
It could take over a year time however so yes, be patient.