tl;dr
Klar imports all your payment providers automatically and gives you the ability to define processing fees in € and payment method fees in %.
How to configure new Payment & Transaction Costs
Go to Settings > Store Configurator in the sidebar
Click on the Shop Datasource you want the transaction costs to be associated with
In the Shop Datasource open the Costs menu
Click the ADD COST + Button
Select Payment & Transaction Cost
Klar automatically pulls all payment providers from your Shop Datasource
Enter a name for the transaction cost setup
Enter the processing fee and payment method fee for all payment providers. (You can usually find these in your Shopify settings.) (Shopify_payments fees are automatically imported - you need to enter fees for other provides e.g. paypal)
By clicking the calendar symbol you can define date ranges to reflect changes in your payment provider fees
Click the SAVE PAYMENT & TRANSACTION COSTS button to save
To edit your payment & transaction costs you can press the transaction costs container.