Allaah says,
(وَإِذَا حُيِّيتُم بِتَحِيَّةٍۢ فَحَيُّوا۟ بِأَحْسَنَ مِنْهَآ أَوْ رُدُّوهَا)
“… and when you are greeted with a greeting, greet in return with one better than it or at least return it in a like manner. (An-Nisaa’ 86)
At the least, you should say, wa ‘alaykumu-s-salaam, ‘… and upon you be Peace!’ If you add, wa rahmatu-llaahy wa barakaatuh!, ‘… and Allaah’s Mercy and Blessings!,’ that is more appropriate and more complete. Hence, the reply is an obligation, and this reply can be via written form, because if you had quietly replied to the salaam yourself, then he would not know whether you have replied or not, and this is the purpose of making sure it is heard or read. Therefore, you should write in your reply, wa ‘alaykumu-s-salaamu wa rahmatu-llaahy wa barakaatuh!, then proceed to respond to his request or thank him for his beneficial post and supplicate for him as response to the benefit he shared with you.