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This will depend on how far it's going, how fast you want it to go, how it's packed, and which method you use to ship.
Probably also what iMac. My 24" M1 iMac is less than ten pounds but the 27" is double that. I really wouldn't trust the US post office with this one.

If you're flying I'm pretty sure you can pack it and put it in checked luggage. Of course put an AirTag on it 😃
 
I want it to be safe and it can arrive late. 27 inch. From Tennessee to California.

I guess checking it in would be the safest option.
 
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