So I have 3rd party plugins that sent some postback data, using manual in here I've able to receive the data and process them without any problem. However, turns out the 3rd party request some particular response to be sent back to them so they know that the postback data is sent successfully. If not, they'll send the postback data multiple times, which I don't need.
So how can I response back from Callback url?
As explained in the documentation you referenced:
If you do not want to write JSON but your own text based output, setting a value with the key “RESPONSE_RAW” in script data will override the JSON output and return this value directly.
Hope this helps,