支付条款在函电中的拟定 — 国际贸易填空题

某出口公司与新加坡买方签订一批户外伞出口合同，金额USD 55,000。函电中的支付条款拟为：买方须于装运期前20天开立以卖方为受益人的不可撤销的即期信用证，货款凭即期汇票连同装运单据支付。请补全该条款英文表述中的空位：Payment shall be made by an ____ letter of credit in favor of the seller, available by draft at sight, to reach the seller 20 days before the time of shipment.