It was already added when I paid the money, so I don't need to pay it again. If you buy it online, there will be details, including fare+infrastructure+fuel+insurance+mailing fee, and finally there is a total amount, which will be paid when you pay. Generally speaking, the invoice for air tickets needs to be mailed. You need to fill in the mailing address. If you don't want an invoice, you don't have to pay the postage.