Option for enable city name only for Shipping Address

Hi,

I am very glad that you guys developed the plugin, was looking for over a year.

I have one request. For our project the shipping address city need to be fixed but the billing address city is free. I can do that by modifying the plugin but if you guys give an option to enable/disable checkbox that would be great. Otherwise in every update the plugin will lost the modified code.

Many many thanks,
Shahi