Yes, it is a felony and in most states nobody would ever check and if they did you did a FTF buy from some unknown in a mcdonalds parking lot.... It's on them to prove you brought it in state so as long as you dont get cought your fine.
NJ is another deal all togeather, I'm not familure with there laws so you should be able to judge if you would get caught easily or not... If it was me... I would ignore them just because thats the mood I'm currently in.
I can say in VA that I have NEVER been asked ONCE by a police officer to see my stamps. I regularly take my machine guns to shows and there with me pretty much every weekend. I have walked past dozens of troopers with a pair of subguns in my hands and they are clueless.
I will say, I wont shread my stamps but I dont feel the need to apply for any more.