求翻译：双方达成的支付条款为，买方必须在2011年10月10日之前开立不可撤销的即期议付信用证，并送达卖方，在装运日后的15天内在日本交单有效。是什么意思？

Reached between the parties, payment terms, irrevocable sight negotiation credit buyer must be opened before October 10, 2011, and delivered to the seller within 15 days of shipment after pay in Japan.

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Both sides achieve the payment provision is, the buyer must before October 10, 2011 draws up immediate negotiation letter of credit which cannot be abolished, and delivers the seller, will be effective in the shipping in the future 15 day intrinsic Japan invoice.

Agreed terms of payment, buyer must open before October 10, 2011 negotiation of an irrevocable letter of credit, and served on the seller, within 15 days after the date of shipment in Japan presented effectively.

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