Postal tax payable = dutiable price * tax rate
The postal tax rate of piano is 10%, and the duty-paid price is determined by the customs with reference to the overseas average retail price of the article. If the customs value is determined to be 20,000 yuan as you said, the postal tax payable is 20,000 *10% = 2,000 yuan.
The tax mentioned above refers to the tax levied in China at the time of entry. As for the Japanese tax that should be paid in Japan's export links, it is necessary to follow Japanese laws.
In addition, the mailing fee is charged by the sender according to the Japanese mailing fee standard and needs to be consulted with the Japanese side.
On the supplementary question.
Electric piano belongs to electro-acoustic instrument, and the tax rate is the same as piano, which is 10%. This tax rate is the lowest among postal taxes.
For the plane consignment fee, it is also charged according to the charging standard of Japan Airlines. In addition, if you can carry it with you or check it, you need to consult Japan Airlines. In addition, generally speaking, the cost of air transportation is much higher than that of sea transportation.