Keycard...?
Michael was able to send himself the collection of random items (his "MacGyver Bag (tm)", as I call it), because they were seemingly insignificant items that wouldn't raise any red flags with security. All except for his ACCESS CARD TO THE HIGH SECURITY SECRET LAB. Surely this would have seemed extremely suspicious to the security personnel going through his stuff. (and he had to assume they would go through his stuff, otherwise he could have just written himself a note telling himself what to do) Making this even more suspicious is the fact that the bag was supposed to contain the items he surrendered upon beginning his contract--since he wouldn't have had a laboratory keycard at that time, the presence of one in the bag would have been a clear indication that the contents had been tampered with.
Also, companies in movies always seem to be conveniently lax when it comes to deactivating the keycards/passwords/user accounts of former employees...
--Ariston
I'm never wrong--sometimes reality just disagrees with me.