Hi @Martinbailey ,
Hyde here from Shopify.
Using any third-party payment gateway (that is, any gateway that is not Shopify Payments) will incur transaction fees. There’s more information on that here. The only means of accepting payments and not incurring transaction fees is to either use Shopify Payments or to offer manual payment options.
Have a look over all that and let me know if anything is unclear, I’m happy to explain more if needs be!
All the best, Hyde.