Ah, last I knew the T-Shirt for stealth shipping with Attitude wasn't free (cost extra). I've ordered from Attitude a few times with the shirt for delivery, seeds in original breeder packs, and always received. Always came in a nondescript Royal Mail shipping bag, sealed up the way they sealed it, with the Customs manifest stuck over it (describing the contents)... it should be sort of evident if it was tampered with or opened by Customs. They would have just re-tapped it in the original package or bag, they don't typically go to any great lengths to make it look like it was never opened.
And when US Customs and Border Protection officially removes/seizes your property, you can bet your ass you are going to get a notice of removal, at least. The notice or letter describes what property was seized and why, including any pertinent laws violated (e.g. Title 21 U.S. Code Section 812), a case number will be provided and a phone number and address should you care to contact them about the matter.
It has been a while since I ordered anything from Attitude though, and I prefer Herbies Seeds. They have guaranteed stealth shipping and they don't make you purchase any additional goods for it.