I had a very similar experience. Same clear tape on the outside. No signature or stamp on the letter.
I noticed a couple of other things:
The package arrived 16 days after I ordered. That seems very fast for a package that was diverted to inspection, inspected, confiscated, repackaged and delivered. I can't believe any US government agency is that efficient.
The letter stated that the contents were "Cannabis Sativus" which is wrong on a couple of levels. First, it is Cannibis Sativa (Sativus is carrots). Second how did the inspector determine what the seeds were. How did they determine it was Sativa (Sativus) as opposed to Indica or Ruderalis? Would they really state the species based on observation. I would expect them to simply say plant material or some such generic term.
I finally found a site that had reviews of many seed vendors (
http://en.seedfinder.eu/seedbanks/reviews/). This company (Original Sensible Seeds Company) had far more ratings where the customer got empty packages with Dept of Ag letters than any other company. Many people were also complaining that when they asked the company to make good on their "Guarantee" they went radio silent.
I can't prove this is a scam but the high number of complaints plus the questionable "letter" is very suspicious. Where there is smoke there is often fire.