How to remove VAT from Royal Mail postage options?

[JKP-123]

Looking for some help.

We have recently went live with our new shopify website. We give our uk customers options of shipping (courier service - eg UPS) or postage (royal mail). 

For some reason VAT is being added onto the royal mail option. As UK Royal Mail (postage) is Zero rated, we know this is incorrect, does anyone know how we can get this changed so we dont charge VAT on Postage (royal mail)