I verified everything and it wasn't a scam; the government was legitimately about to take me to court or start garnishing my wages or something like that. They said that UPS or FedEx had told them I imported the equipment but never paid import duties.
I got ahold of the case worker for my case and told them it must be some error. After a week or two of back and forth, we finally figured out that it was for a laptop that a user in Canada had shipped back to me when they left the company, and it had been mis-categorized and valued on the import documents. Since we had purchased the laptop in the US in the first place and shipped it to the user, the case was dismissed and I didn't have to pay anything, but it's always a mini heart attack when you get a letter from Uncle Sam saying you owe thousands.