Hi, @Gc4. I'm happy to help you out with this! Can you tell me more about the issue customers are having at checkout? Is this only happening for customers from the Channel Islands or have you noticed this with other customers as well?
In order to allow customers from different regions to checkout you'll want to make sure that all of the countries and regions you ship to are included in your shipping zones, and that each shipping zone has available shipping rates. For the Channel Islands specifically you'd want to make sure you've added 'Jersey' and 'Guernsey' to your shipping zones. This guide on setting up shipping zones and this guide on setting up shipping rates can help further. Before you do this, you'll also want to make sure these regions are included in active markets since countries and regions can only be added to your shipping zones when those countries and regions belong to a market. This guide on shipping zones and Shopify Markets can help you do this.
Please let me know if you have any questions!
Erin | Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog