Add custom field to shipment plugin

[keep]

I want to write an own shipment plugin and I need to add a custom dropdown field with some options to the shippig method. The dropbox values are in a separate table.
I need to store the user's choise and display it in the invoice mail and on the FrontEnd.

Is there any way to do it?

[attachment cleanup by admin]

[PolishedGeek]

Will the shipping price vary by the options selected? If so, has to be a shipping plugin as you stated, and yes, you can have a plugin pull from a configured or pre-populated table.

If the shipping price won't vary, then you could use a custom order field, which would be simpler.