Customs usually only hold flashcarts, in bulk, from the point of origin. So, Hong Kong in many cases, will stop shipments that are obviously mass mailings for flashcarts or other restricted stuff. I think its only happened once or twice, and it may not have even been shipments to consumers but resellers, or import/export from factories.
Usually, the only reason a flashcart gets held up from customs is from random inspections and other items with the cart that cause concern, like batteries, lasers, knives, etc.
If you're just ordering a flashcart, by itself, your odds of having customs seize the item is about equal to many lotteries.