If you don't care about aerodynamics, which as a commuter with panniers is something I don't think too much about, then you should check out fishing clothing. Columbia PFG is what I'm familiar with, but there are less expensive brands, too. I think Magellan is either Academy or Bass Pro Shops. They make light, wicking, breathable shirts that are traditional button up style. Usually with the ability to roll up the sleeves or open a vent on the back, and also with extra material on the collar to fold it up and block sun on the back of the neck. They don't have any reflective, but they come in light colors.