Well for shoulder presses, you could always grab any object and just lift it over your head until your arms are locked. So even, say, milk jugs, boxes, whatever.
As far as why the biceps and traps may be getting developed, here's my theory. You say you do heavy lifting at your job, so what happens if you pick up any object off the floor, your back and legs do a lot of the work, but your traps get worked very much, too. They especially get worked with any kinda shoulder shrugging motion. As for the biceps, you could be sorta "curling" whatever you're moving, as any kinda odd shaped object, to lift it level (ie, in my case, a desk) you generally have to curl your arms a bit to grip it, and especially if you gotta move with it, your arms will be in a constant partial curl.