To create the select list for the user to select the service type you will make use of eCart. There are snippets for the service type select lists. You will need to incorporate these into your checkout page, then update the shipping quote server behavior to use this new value on the confirm page.
You will also need to have hidden form elements for these select lists on the confirm page, the elements should have the same name as the select list and the values should be set to the posted select list.
There is one more part to this which is selecting the main service provider. Since this is determined by the globals variable $WAGLOBAL_Shipping you will need to update the value of this session variable based on the user selection. One way you can do this would be to have the two select lists on the page and have an empty value for the first option. You can then just check to see if there is a value for the select lists for that service, if there is then set the global variable accordingly.
If you need any help with this last part just let me know and I will be glad to help you out.
