所以应该再付$669.5给我们公司！
直译会比较丛虚扮生硬，誉纳以下渗灶是意译：
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.
The amount on this invoice already included a 5% discount, but the payment that our company received contained an additional 5% discount. Please remit another $669.5 to our company.
This invoice amount has lost a 5% discount!
But we actually received the amount is decreased again times a 5% discount!
So should pay again 669.5 to $our company!