When you get to the pp_confirm page the default shipping quote server behavior should already have gotten the quote from the API. You might allow users to change this by adding another small form into the page, the form would consist of a service type select list, and a submit button. You will need to adjust the shipping quote code to look for this posted element, and if it is not found use the default value.
When you do it like this you should be able to submit a new value for the shipping quote to work with and get a new quote back.
One thing to look out for with this though is that if you have the page submit to itself you could trigger some of the other server behaviors, so you may need to adjust the other triggers to only fire when the specific submit button is pressed. If you have any problems with this post back with your pp_confirm page and let us know what you have going on and we will help with whatever we can.