"instant reply" is correct and usable in written English.
You can use it when referring to a swift response, for example: "I was delighted to receive an instant reply to my query."
"Nothing," came the instant reply.
"Yes," came the instant reply.
A firm "no" comes the instant reply.
"I think it is the authenticity," was his instant reply.
As this situation is extremely urgent, we request your gracious instant reply.
When Len asks "Wass yer name?" Pam's instant reply is a "Yer ain' arf nosey".
But Ms. Berk could send questions, and Ms. Sigler could tap out instant replies.
"In diplomacy, as you know, we don't get instant replies," he said.
When she received instant replies from colleagues, she realised others were having the same reaction.
Instant replies to emails are expected, even if sent in the middle of the night or the weekends, and employees are apparently actively encouraged to secretly funnel their back-stabbing or praise through Amazon's "Anytime Feedback Tool".
Conceived by Frank G. Dunham Jr., vice president of the Millers Insurance Group, the clearing house for information essential to in dustrial expansion and new site selections ultimately could employ telephone lines for nearly instant replies to worldwide queries from large and small businesses.
