This is a known dirty industry trick to draw in bookings for their cars. I was tricked before. After 3 biddings, they tell you they cannot get the COE at the price you booked and asked you to top up. If not, they will just continue to bid without increasing the bid until the contract runs out. By that time, you would have lost 3 months!

Make sure when you sign the purchase agreement that you are gauranteed the COE and car without further top up.