How can I get prepaid labels from Royal Mail for customers to print?

Hey guys, the title pretty much says it all!

I want customers to be able to generate their own prepaid labels to print and then take to the post office. I’ve seen companies use them for returns, but I will be using them for customers to send me their devices to repair/sell

I am based in the uk, so any UK based companies that offer this kind of service would be fantastic!

Also using Shopify, but I’m guessing this is a bit irrelevant as it would be an external link!