You should reply to let the person who sent the invitation know whether or not you plan to go.

你应该回复，让发出邀请的人知道你是否打算去。

youdao

Sandy's mother told her, "When you get an invitation, you should reply as soon as possible."

桑迪的妈妈通知桑迪：倘若你收到了请柬，你要尽快答复。

youdao

If you get a "friend" request from an old flame, talk with your spouse about whether you should reply and how that will make your spouse feel.

当你从老相好哪儿收到好友请求的时候，还是跟你的配偶谈谈是不是应该答复，该怎么回复吧，想想你的配偶的感觉。

youdao

" And in such case, "you should destroy this message and kindly notify the sender by reply email.

WSJ: Email Disclaimers Proliferate

You know, major manufacturers should just start handing out class action lawsuit form letters and business reply mail envelopes with their various electronics, just to make these things easier for everyone involved.

ENGADGET: Microsoft hit with lawsuit over Fall Update woes