The message is to a pre-arranged place out in the Borderlands somewhere, there will be someone waiting for it. I leave the details up to you. The only thing is that where she'll be going (if she goes, again up to you), is a staging place for the Prince's army -- not that it screams 'recruiting army here' or anything like that, of course.
If Sybra decides to go, they'll pay her her fee on receipt of the message and give her one to carry back. If she decides, for whatever reason, to open the messages, the first one is about Karadur's arrival and the other goings-on in AF, and the second one is an alert to the men to keep their eyes open for fugitives (Alan and Kynan) with their descriptions.
From the spy-hut, there will be others to ferry the message back to the Prince and they won't send it off until she leaves.
Let me know what else (if anything) you want to do and I'll tell you the result. It's approx early evening in AF, so, going to the hut will bring Sybra back, at her fastest speed, sometime around midnight.