First off the letter then sent you is to advise you it has been siezed, they will offer you an oppurtunity to recieve your goods but you have to show documentation that it is legal for your intent and purpose, Which it probibly is not.
They might make another chance at delivery to your address in which they will ask you for a signature of reciept, DO NOT DO THIS !!! Within minutes of signing for your package you will be raided by DEA and customs, They will try and charge you with delivery and or recieving a controlled substance by mail. They will try to nail your ass.
If you are stupid enough to sign for the package, be prepared for the whole good cop bad cop bullshit. When it goes in front of the judge they will have the charges trumped up to the equalivent of possesion of a synthetic narcotic, delivery of a controlled substance with intent to distribute, depending on the amount.
You could probibly ride it out and keep post poning the hearings but I would recommend you get a good Attorney, one who deals in this, do not opt for the public defender, he's the equalivent of a piss stain on a jail house toliet.
Oh, By the way good luck