I ordered my seeds from amsterdammarijuanaseeds.com. All they do is sell nirvana seeds at a higher price. Meaning i got the same white evelope you did. Mine had a card folded up inside and two bags of seeds taped to it on the outside of the card. Wasnt very hidden inside the envelope, and mine made it through. If your seeds were taken by customs you would have just gotten a letter from them not your package.
Somebody inside the mail department got you, and that really sucks
Maybe if you plead your case, nirvana will help you. I hope they do