you can add shipping manually after running the checkout wizard.
on the confirm page, go to the Server behaviors panel and click the plus button, then select ecart -> Shipping then selct the provider you wish to use.
You then need to add a shipping rule in the shipping tab for that provider.
Both triggers and calculation list have built in options for your provider.
to offer a choice of shipping requires some hand coding to accomplish.
you need to add a select list to the checkout page, and also a hidden form element of the same name on the checkout page:
<select name="shipping" id="select">
<input name="shipping" type="hidden" id="shipping" value="<?php echo((isset($_POST["shipping"]))?$_POST["shipping"]:"") ?>" />
the add a set session value behavior.
Set the trigger to any form post
set the name to shipping
click the lightning bolt next to value and select the shipping form element
Now, when creating the shipping rules, add an additional trigger:
based on session variable value
for fedex, the trigger would be created as:
If the Session variable 'shipping' has a value of 'fedex'