On our store https://solonz.co.nz/ a user can input an address and it will auto fill their post code using Shopify's default address validation functionality.
That post code aligns with a location in New Zealand and a relevant post code.
That post code represents a territory and a charge that is related to that post code.
If a user wants to, they can change that post code to one that will have a lower delivery charge. For example post code 3173 is rural and post code 3112 is in town.
We are wanting to make it so that the user is not able to edit that field, which is auto populated to ensure that they are paying the correct amount for shipping.
What solutions do you think could work to restrict a user from spoofing the post code field?