Plugin for extending shipping informations

[maxxer]

Hi.
I need to develop a plugin to extend shipping information. I simply need to add a checkbox in the same page where the user selects the shipping method.

I started looking at the vmShipping* but I understand that's for defining a shipping method. Then I found the vmCustom plugin, but I cannot understand if it's a generic plugin or just for creating custom fields.

Sadly I found the documentation a bit light on the subject, so I'm having some difficulties in understanding where to start.
If anyone is willing to help...
thanks