I know the feeling, if it helps you at all its not really anyone´s fault.
Ive had that several times with different parts. They are usually malfunctioning right away or briefly after the usage has started.
I dont think they will charge you extra. As long as they can replicate the problem there as well, they are simply going to swap new Mobo. I dont see it as a reason to swap stores, as its just matter of bad luck and you are as probably of getting new malfuntioning part from the new store as you are from the current.
Do you have to send it by mail?
PS. Is there front plug? (because there is a real possibility that it might work without any problems. I had Mobo like that I didnt want to go through returning by mail and the wait. I used the mobo´s soundplug for several years without any problems).