No actually they just cram it in the box
If the package won't fit in your P.O. box the clerk will leave you a key in your box and you can retrieve your package from a parcel locker. If the package is to big for a locker they will leave notice in your box and get it for you when you come to the counter.
I get things occasionally that are too big. What they've always done for me is they put it in a larger box, and they leave a key in my box, that has the box number the key goes to on it
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