From ATF:
Who is affected by the Federal explosives and regulations?
"The Federal explosives law and regulations affect all persons who import, manufacture, deal in, purchase, use, store, or possess explosive materials. They also affect those who ship, transport, cause to be transported, or receive explosive materials. Explosive materials are any chemical compound, mixture, or device, the primary or common purpose of which is to function by explosion. The term includes, but is not limited to, dynamite and other high explosives, black powder, pellet powder, initiating explosives, detonators, safety fuses, squibs, detonating cord, igniter cord, and igniters. (Powder and primers in red)
Persons who manufacture explosives for their personal, non-business use (e.g., making fireworks to set off on your own property or mixing binary explosive components to remove a stump in your own yard) are not required to have a manufacturer’s license. However, no person may ship, transport, cause to be transported, or receive explosive materials unless such person holds a license or permit."
I was one of those "really bad Customs Guys" I just read about, and there is a bit of truth in those statements. Being knowledgable will protect you from most things. Attmepting to 'hide' stuff will only make it worse when discovered. Once it is discovered and a record made, it will never go away. A guy asked me once how to get his name out of the system. I responded that if his grandaughter brought me a copy of his death certificate, I'd think about it. But in all honesty we wouldn't. Saving $5.00 or even less for a can of gun powder can't be worth it.