Screws break, it happens. Whether they're from the hardware store or from an airsoft part. It has happened to me several times using screwdrivers by hand.
I've never had a problem with TMC customer service and will order from them again in the future.
I honestly can't see fault on their part, they shipped you the part quickly , you got it, then broke it. I won't comment further about intending to use it on RS, the posts above mine have said enough on that.