Setting shipping rate

[pooja]

Hi,

I want to set a shipping rate to 'X' only if order total is less than 150. Otherwise shipping should be free. I know that I can achieve free shipping above 150 by configuring 'shipping' in 'Order Totals' module in admin portal and that works fine.
      But if order total is less than 150, it gives an error in validateOrderTotals() that no shipping module found. Where can I set 'X' amount for order total less than 150?
Requesting for a guideline.

Thanks in advance.