If you want to specify a default shipping method on the checkout form, you just need to set the
default attribute on the
shippingMethod form field as follows:
[MM_Form_Field name='shippingMethod' default="FLATRATE-2"]
In order for this to work correctly make sure that you place the SmartTag between the
[MM_Form type='checkout'] and
default attribute expects a valid shipping method key. To locate the shipping method key go to Checkout Settings > Shipping Methods and then hover over the icon next to the shipping method you want to lookup the key for.