Verin could not give Mat that letter unless she convinced herself he wouldn't open it before the 30 days were over. One of her BA oaths prevented this. If she was wrong, all the better - the letter hardly had exacting demands anyway - but she had to be convinced that she was right.

So it follows that Verin was precisely counting on the fact Mat would refuse to open the letter early because he would be much too afraid of her "AS tricks" to risk being promised bound to follow an AS' orders he knew nothing about. Despite saying the exact opposite (which only made Mat more suspicious) Verin made it all sound like the letter had to contain instructions that Mat would not willingly follow otherwise. Why else not ask him to follow her instructions in person, if it was anything Mat wouldn't mind doing? So Mat postponed opening the letter until the delay was over. Mat would normally have then done exactly what Verin expected: open the letter to see what he "escaped" from the minute the 30 days were over.... and wonder madly why the hell Verin couldn't just pass him this information instead of playing AS games with him (but Mat's clever enough to have puzzled out that Verin meant him not to open the letter until the end of the 30 days, if not quite why she wanted him to do that). Verin didn't outwit herself or misread Mat. She was the victim of missing important information that she could not factor in. She couldn't possibly foresee that Mat would have a reason not to risk opening the letter in case what it contained might force him (morally) to remain in Caemlyn to follow her instructions even if the 30 days were over, nor that Mat was under pressure (Thom's) to go to Ghenji. Had she known that, she would have put it quite differently to Mat, no doubt.