I've only seen two articles claiming evidence of physical tampering - like a hoax about Dell from 2005 [1] and Bloomberg's dodgy story about spy chips from last year [2] - neither of which seems truthy, and neither of which involved mail interdiction.
(Of course, it's widely suspected mailed hardware can be tampered with, but most of the claims/speculation I've read has been about targeted tampering, not dragnet)
[1] https://www.snopes.com/fact-check/keyboard-loggers/ [2] https://techcrunch.com/2018/10/04/bloomberg-spy-chip-murky-w...
On what legal authority?
The border search exemption is about searching, not tampering.
The issue is more that border agents are taking advantage of a law written before the age of computers to use powers the founders probably never intended.
I seriously doubt that there's any legal basis to bug a computer just because it was shipped internationally.
So if the threat model is being asked for your password, not being present when the device crosses is a good mitigation.
(If you've done enough bad things they government is targeting you specifically, YMMV)
is that really true? I would expect that they can just perform hardware tampering when you're not present when searching in mailed items, but given that customs might use racial profiling to target you at airport, you may be better off mailing the computer securely, such as using tamper-proof or tamper-evident stickers/bags.