If you send packages from England to America, or vice versa, do they search the packages in transit?

I've ordered a number of records from England, recently and, as far as I know, they haven't been searched. Some of the boxes, from what I recall, were factory sealed in that, to open them, you had to zip off some of the cardboard packaging to get it open. The same kinds of zips on FedEx boxes. If someone searched inside, the zip would've been destroyed.
I've also had people from around the world send me letters and money for me to search for records to mail them and, as far as I can recall, none of the stuff got searched.

If you send packages from England to America, or vice versa, do they search the packages in transit?
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