Uh, if they can sit outside of the store in wait for the robbery to occur, they had enough manpower to do what has been suggested and have an undercover officer in the store in place of the clerk.
If they have enough manpower to give out speeding tickets and handle ANY police work at all, they have enough manpower to handle a possible armed robbery and homicide.
It just occurred to me that perhaps they just hadn't finished the donuts they purchased in the store just prior to the informant dropping off the perp.