Hi,
You could also change the city custom field to a dropdown. But shipping methods can't be restricted based on the city field anyways, so even if we were to develop a mechanism for the city selector to refresh based on the country / state selected and have city zones in HikaShop, it would also require the zone restriction of shipping methods to support that extra field, which isn't the case.
So, basically, doing this would require a lot of work on our end.
You could use the post code instead of the city name. Shipping methods can be restricted based on the post code and usually cities and post codes can be used to restrict to the same areas.