We do not have a guide for the scenario you have described. There are shipping options snippets with eCart but they are for the specific provider. If you go to snippets > eCart > shipping you will see the various snippets for the shipping. You can use the snippet for a particular provider, this will add a select list for the option you choose. For example of you choose the FedEx Services snippet you will have a select list on the page for the user to select the service type.
You will then need to add in a set session server behavior on the confirm page to store the posted value in a session variable. You will also need to add a new hidden form element to the form on the confirm page, you should give it the same name as your service select list, and set the value to the session variable you set earlier for this element.
Once you do this you can edit the shipping server behavior and reference the session variable for the service type.
It is possible to create the scenario you describe but it would be a custom implementation and would require you to start with the shipping server behaviors for each of the providers you would like to offer. Please post back with some more questions about this and I will offer advice on any part that I can.
