on the checkout page add the dropdown list:
<select name="shippingOption" id="shippingOption">
<option value="">Select Shipping Option</option>
<option value="Next Day">Next Day</option>
</select>
On the confirm page, add a hidden form element with the same name to capture the value posted from the checkout page:
<input name="shippingOption" type="hidden" id="shippingOption" value="<?php echo((isset($_POST["shippingOption"]))?$_POST["shippingOption"]:"") ?>" />
Then add a set session value server behavior to the confirm page. On the server behaviors panel, click the plus button and select eCart -> General -> Set Session Variable.
Set the name to shippingOption
set the trigger to Any Form post
For the value, click the lightning bolt and select the shippingOption form element.
Finally add a new shipping charge on the shipping tab of the eCart object.
Set the trigger to "Based on session Variables value":
Id the Session Variable 'shippingOption' has a value '=' to 'Next Day'
for the calculation, use a flat rate of 5.00