The mail I sent last time was of my misunderstanding.
I'm sorry, and please excuse me.
I'll pay the bill immediately. When the payment is completed, I'll mail it to you again.
I misunderstood you and sent you an email. I'm sorry for that.
I'll pay you as soon as possible. When I have completed my payment, I'll send you an email again.
なお、最後のemailにtheをつけてしまうと、はじめに送ったメールを再送することになりかねませんのでご注意を。
以上でいかがでしょうか?