Someone on the radio suggested that he sell the cap to his wife for $1 and then ask her to bring the cap back to Australia

kpc

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If he simply took it on the plane as carry-on I can't imagine anyone would have cast a second glance at it.

OR, something small like that mailed Registered Post would arrive without incident from the UK. There is no value declared on Registered Mail sendings providing weight does not exceed 500g. As long as he raised an all-risks cover note on it, he is covered for any loss or damage in transit.

$42,000 GST on a mouldy cap is pretty nuts.