Additional fee for payment method

Where do I find the settings for additional fees for payment methods? I've been clicking arount among the payment methods and can't find anything. Thank you!

Go to a payment method details page, then 'Configuration' tab

But not all have it... Purchase Order for example doesn't have anything on the Configuration page.

Yes, not all have it. If you need to have additional payment fee for Purchase Order, then it'll require some customization

Open NopSolutions.NopCommerce.Payment.Methods.PurchaseOrder.PurchaseOrderPaymentProcessor class and replace

public decimal GetAdditionalHandlingFee()
        {
            //override if payment method requires additional handling fee
            return decimal.Zero;
        }

with

public decimal GetAdditionalHandlingFee()
        {
            //override if payment method requires additional handling fee
            return 10; //10 is your Purchase Order additional payment fee
        }