I've been working on the 2-wheeled Arduino robot kit, most of which (the kit, the Arduino Uno, and the Motor Shield) was purchased from Makershed.com. Once I got it assembled and running, I discovered one of the wheels was warped, causing it to drift sideways when it should have been moving straight. I emailed their customer service, requesting a replacement wheel. To my delight, they responded the next day saying the replacement had been shipped. That was three or four days ago.
Today, I got the package. First thing I noticed upon picking it up was that it felt a bit heavy to just be a plastic wheel.