I weigh about 205 and only carry around 20 pounds of stuff, but the only time I've broken spokes is when I've managed to put the chain into the rear wheel. I generally ride with 32-spoke wheels and 700x25/28 tires, though I did finally go with 36 spokes on the last wheel I built for my commuter. I used 24-spoke Mavic Aksiums on my road bike for a couple of summers, and they held up without an issue.

It possible that your LBS left a flaw in the wheel. It's also possible that the components just weren't up to the task. It's hard to say. Riding style makes some difference. Wider tires would also help, though perhaps you can't do that with your current bike.