Por que "Thank you very much"?

Avatar do usuário Henry Cunha 9920 2 17 177
One thing is certain: You can say just "Thank you" or all of "Thank you very much". In each case, nothing more and nothing less.

As to the use of "too" in this expression -- in the same form and with the same meaning -- no. The meaning would change to "agradecer demais", and so would only work in this kind of utterance: "He was thanking her too much, she felt, considering she had done very little for him."