Hi, @Magnoliker .
For more help with this it’s best to speak to Braintree directly as they will have the most information on their payment gateway to be able to help further. You may also want to review other payment gateways or payment providers you can use in Jersey Channel Islands. If you type ‘Jersey’ into this page you can find a list of payment gateways available in your region that you can use with Shopify.
Please let me know if you have any questions.