How to add a custom suburb

Hi,

It should be a custom field of the table address and of the type text.
The default value should be left empty.
You should indeed turn on the front end display.

You will also want to change the address_template file via the menu Display->Views in order to add a tag for your new custom address field.

No, it's in address_template. Please make sure that you change all of them.
You have one in the front end and one in the back end.
If you open an invoice on the back end, it's the back end file which is used.

Also, make sure that you edit the back end view file for the template of your back end, and not for the template of your front end. Otherwise, it won't do anything.