Solved! Go to the solution
We use Royal Mail Click and Drop and our account has synced up to our Shopify Store. For some reason, a few month ago the app was no longer available as an extension however, it is still synced up and we simply log-in to our account online: https://business.parcel.royalmail.com/ and it has our orders ready for us to apply the postage and print the labels. Not sure if that helps you or not?
This is an accepted solution.