Please keep this info for future case na ka. I forgot to think about it at the first place when I suggested N'1 to ship direct from USA to the customer, sorry ka. For this case, here's my suggestion to write to the customer.
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Dear Mr.xxx
We asked shipper to put $100, but they couldn't, giving the reason that the price is too low, and their Customs would question to them because it would be too much different from their other products and price range they normally export.
We are sorry for that and hope the declared value at $371.20 would be ok with you, as it helps save some duty (if any) already.