Why are there other rates displaying at checkout?
If your customers are seeing other rates during checkout that you didn’t configure with Postcode Shipping, you might have some rates setup in your Shopify Admin you do not need.
If you've setup rates directly in Shopify Admin, then they will display during the checkout process alongside the rates you have setup in Postcode Shipping. The app will not block other rates configured in your Shopify Admin.
It’s worthwhile double checking your Shopify Admin and deleting any unnecessary rates. See our article on Deleting Shopify's default rates.
If no rates have been configured anywhere else, then you must be targeting that specific postcode in one of your regions.