domain mapping issue

hi there,
i have finally managed to get the domain mapping working.
however, i have a shop on one of the sites, and the shopping cart doesn't work while the domain mapping is activated. once i turn the domain mapping off, the shopping cart works just fine.
if you could help me to sort out this problem that'd be great.
thanks,
Adam

http://www.pauldowningmusic.co.uk/shop/