Hi @james177 ,
There are a lot of apps that helps with international selling. Choosing the right one depends on your exact requirements. As a first step, you can look into this Shopify International shipping guide to get a better understanding regarding the selling process. Regarding the apps; may I know which carriers you are planning to use? For all top carriers, you can check out our Multi Carrier Shipping Label app that helps you integrate most carriers and seamlessly handle the international selling process.
You can also look into some of the other apps that helps with international selling here. Now, if you are looking for apps that provide you with their own discounted shipping rates, you can check out some of the apps here.