I saw the total on the invoice and I was dumbfounded. All I could do was put my head down in shock.
この場合、次のような言い方ができます。
ーI saw the total on the invoice and I was dumbfounded. All I could do was put my head down in shock.
「請求書の総額を見て茫然自失となった。ショックでうなだれるしかなかった」
to be dumbfounded で「茫然自失となる」
I was stunned. とも言えます。
to put one's head down で「うなだれる」
ご参考まで！
Looking at the total amount on the invoice, I was speechless. There was nothing for me to do but to droop my head.
ご質問ありがとうございます。
この場合「[請求書](https://eikaiwa.dmm.com/uknow/questions/36500/)の金額を見て茫然自失となり、うなだれるしかできない」は英語で「Looking at the total amount on the invoice, I was speechless. There was nothing for me to do but to droop my head.」と言います。
ご参考になれば幸いです。
カナダ 
アメリカ合衆国 
