ready reply; quick answer
Common mistake for 二つ返事 (ふたつへんじ), which is the correct expression for an immediate, willing reply. This form is often used erroneously, but the intended meaning is the same.
See also: 二つ返事
Using 一つ返事 for 'accepting immediately' is a mistake; the correct phrase is 二つ返事.
A mistaken variant of 二つ返事 (ふたつへんじ), literally 'two replies', which refers to answering 'yes, yes' without hesitation. The form 一つ返事 likely arises from confusion with the number of replies.