支付条款在函电中的拟定 — 国际贸易填空题

某出口公司与意大利买方签订一批鞋类出口合同，金额USD 80,000。函电中的支付条款为：买方须于装运期前20天开立不可撤销的即期信用证，以卖方为受益人，货物装运后凭全套正本提单付款。请在催款函英文表述中补全空缺：The buyer shall open an ____ L/C in favor of the seller ____ days before the shipment date, payment to be made at sight against full set of original Bills of Lading.