Fedex Customs Dispute…legal Help Necessary?

I sent a package (men’s shirts/trousers) from Japan to a customer in the US in December. The correct value on the customs form should have been ¥130,000 JPY (roughly $900) but I mistakenly forgot to manually change the currency code from USD (the default) to JPY which resulted in the value being marked as $130,000 USD. The mistake was quickly noticed by the Japanese staff once the package arrived at the local sort facility. I promptly sent a digital copy of a revised customs invoice with the correct currency code and was assured that the correction was confirmed and the package would be cleared for departure.
However, nearly six months later, Fedex US is still trying to collect nearly $22,000 USD in customs duties as well as additional processing fees from the recipient. I have provided them detailed confirmation of the communication with Japanese staff as well as copies of the corrected invoice and dispute form filed when they initially tried involving the customer.
Could anyone offer some advice as to how to best resolve this? I would like to resolve this on my end (rather than the customer) and I would guess that US small claims court might be the answer, but with me being in Japan, I don’t know if that is an option. This is very frustrating because, as the Japanese agent quickly took note of, $130,000 USD for a 2kg package containing a shirt and trousers is clearly absurd and obviously a mistake.
Thank you for any help that can be offered!
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