どうもありがとうございました。

Dōmo arigatō gozaimashita.

Thank you so much (for what you did).

Word for word

“Thank you very much for what was done.”

Politeness: Polite

です/ます style. The safe default with strangers, shop staff and colleagues.

When you would say it

After a favour is complete: help given, a purchase made, a visit.

Worth knowing

Past tense because the act is over.