求翻译：买方应通过一家卖方可以接受的银行于装运月份前30天开立并送达卖方不可撤销即期信用证，有效至装运日后15天内在中国议付是什么意思？

Buyer shall be accepted by a seller bank 30 days prior to the month of shipment and delivery the seller to open irrevocable letter of credit, effective within 15 days after shipment to negotiate in China

正在翻译，请等待...

The buyer should the bank which may accept through a seller previous 30 days draw up and deliver the seller in the shipping month not to be possible to abolish the immediate letter of credit, effectively to shipping in the future 15 day intrinsic Chinese negotiations

Buyers should be shipped by a bank acceptable to seller in 30 days before the opening of the month served and the seller an irrevocable letter of credit, valid until 15 days after the date of shipment in China during negotiation

The buyer should open in 30 days and send to seller's irrevocable sight letter of credit before shipping month through a seller's acceptable bank, will negotiate in China in 15 days effectively in the future to ship