Very small amounts are rarely intercepted. Letters are far safer than packages, and smaller packages are far safer than larger packages. Customs and USPI are not looking as hard at things that are clearly not large shipments of drugs or money or bombs. Sending slower helps a lot as well, packages are scrutinized to an extent that correlates positively with shipping speed and package weight. At the extreme safety levels for each of these, interceptions that are not based on intelligence are extremely rare, even in places like Australia. This might change to an extent as customs and USPI learn more about swarming, they have already recognized that traffickers intentionally break shipments up into tiny groups of less weight to increase the chance of getting large amounts through customs, so they might try to catch that more. But they are probably also limited by the amount of the mail, if a letter doesn't stick out at all from the average type, and dogs can't hit on what is inside of it or it is perfectly vacuum sealed with no trace contamination, I think they will have a hell of a time to detect it when it is in its initial inspection to know that they want to do more time consuming testing on it.