Provinces for Canada are a dropdown. CBSubs calculates its tax based off the invoice address, which the user can change/specify on the basket page. You could make the invoice address mandatory within CBSubs > Settings > Display > Invoices to force them to complete the invoice address fields.

You won't be able to synchronize 2 different fields to CBSubs unless you do the synchronization probably with CB Auto Actions. You'd then need to use CB Conditionals to show a text input if the other option is selected. Best option is probably to have a CB Query Field and its Query Select fieldtype to query for the provinces directly from CBSubs so they can select one that directly matches CBSubs. Example of this usage as follows.

www.joomlapolis.com/forum/153-professional-member-support/230586-cb-field-select-country?start=6#268337

I guess I should start with a more basic question- how do I set up state/province drop downs and country fields? Was my screen shot above okay for the province/state drop down?

You either need to manually setup the select fields or use CB Query Field and its Query Select fieldtype to generate a dropdown of countries and/or provinces from CBSubs database tables.

The country drop down will have to be programmed as you have it edit.front.invoice file? That might take me a while as I have to copy and paste each country in there.

The value synchronized from CB must match the value CBSubs expects otherwise it won't work. See my above reply to automatically generate a select dropdown from CBSubs own database tables to ensure it's exactly the same.