My day job is buying and selling from around the world; common to work with five continents in one day.
Usually the shipping company acts as the broker for you, and your duties and fees are part of the price of shipping. On higher value shipments, it's also very common for shipping folks to contact me for an additional payment before items are delivered to me. Sometimes even months later, when customs finally caught up with them.....
I've also had customs call me, asking why zero value was attached to a shipment coming back to me, but the same shipment had value when it went out. Lady, it's zero value because it's a faulty part being returned. My apologies, you're right, I guess half a pound of scrap metal does in fact have SOME value..........
I've also had customs seize shipments and hold them for six months while they figure out what they're doing, before releasing the shipment. Customer is long since pissed off and gone by then, nothing I can do with the parts, sale is ruined. How this isn't a 4th amendment violation, I don't know, and when I asked who I send the bill for the hassle and lost sale to, I got no response. It would be tough to decide if customs or the IRS gives less of a **** about the problems they're causing you while "just doing their job."
There are ways around all of this, but we run 100% above board, and pay every single cent of tax and duty that we owe. There's enough red tape and hassle involved already, we don't actively try to find more trouble.