i,also, have a 3.2mile ride each way!
i use bikenash top loading rear panniers (about 2200cu/in total), +/-$35.00. i double (or tripple) plastic bag inside contents like briefcase etc) when it rains, gear does stay dry!
i do have a rack-pac too for lunch, tools and nite lights.
i would NEVER attach any gear to my back!
enjoy pannierism on your commutes!