jiǎn dān huí yìng jiē yī jù chǎng jǐng jiù gòu zì rán bù bì xiě chéng zhèng shì gǎn xiè xìn

简单回应、接一句场景，就够自然，不必写成正式感谢信。

A simple reply and a situational sentence are natural enough; you don't need to write a formal thank-you letter.