这个发票的金额已经减掉了5%的折扣了!但是我们实际收到的金额却再减了次5%的折扣!

直译会比较生硬,以下是意译：
The amount due on this invoice already reflected a 5% discount.However,the payment we received was 5% below the invoice.Therefore,an additional $669.50 is due to our company.