Hi Jason and thank you so much for the help you have given me.
I have added the checkbox to the checkout page with the correct values and the hidden field to the confirm page, again with the correct values.
Unfortunately, I am still stuck on one small point and that is setting the session value. For some reason when I click on set session value server behavior my form element does not appear anywhere in the list. I get the usual WA sessions appear aswell as my ecart object and the local checkout response elements but not my 'nextday' form element. How would I manually enter the data in the trigger field and the value field.
I am coding in ASP rather than PhP and I know from previous experience that the form element would normally appear if the page was PhP but not necessarily in ASP.
With your help, I feel I am very close to completing my shipping charges so hope this will be the last request for help.