So, the Reach have Jaime under constant surveillance. He's up on their video screens at all times. How did he get to mail the letter without them noticing?
Judging from the Negotiator's confused reaction, the Reach didn't even know what a letter was; they were so advanced that him putting a piece of paper in a box didn't even register to them as a form of communication.
Also, the Reach may have Jaime under constant surveillance, but the Negotiator is not constantly watching him. The Negotiator is often busy doing other "take over the world" stuff, and the minions he's got watching Jaime were probably just looking for key things such as meeting with other superheros. Also, the Reach are primarily watching him by tapping into his phone and e-mail (which can give an automated alarm if certain words/phrases pop up) NOT by watching him 24/7
Also, I have a hard time seeing how the scarabs fit in to the Reach's plans as outlined by Jaime in issue #22.