Those who import goods: Why am I not being billed for customs/duties?

I have a feeling I'm going to get hit with a big bill soon..

I do not currently have an LLC, I operate and pay taxes as a sole proprietor using my SSN. All imports are shipped directly to my place of residence, and my place of business (my house).

Sometimes when I import goods, I'll receive a letter from Fedex or DHL saying I owe x on duties, and I pay it. Most of the time, though, I receive nothing.

I assume this is because you only pay duties when reselling? I'm not sure if that's a fact, but that's what I'm assuming from what I've gathered so far. Because the goods are shipped to my home address and I'm not an LLC, maybe they believe it's for home use and I dont plan to resell, so I'm not paying duties? I dont know.

I just know that I SHOULD be paying duties, and I'm not being billed for it. What do I do? I dont really know what the problem is. I am asking reddit as a place to start proper research and get a few leads. Importing is very complicated and I just want to ask those with experience first.

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