If the back wheel is not turning, then either the motor or the batteries are banjoed.
If the repair bill is in the amount you specify, I assume it is the motor to fault.
DO NOT pay for a repair so soon after purchase, as a 7 day warranty is, at the very least, dodgy...
Go to the local citizens advice office, or in the UK contact www.consumerline.org, and tell them the problem. They love this type of complaint and will NOT allow shops to get away with dodgy deals.
Davymac
August 2010