No cost for coupons

Hi all,
We are going to start selling e-tickets soon. No matter how many tickets you buy at once, you only pay € 0,29 costs (‘Transactiekosten’). I managed to do that with this piece of code in functions.php

add_action( 'woocommerce_cart_calculate_fees', 'bbloomer_add_checkout_fee' );
function bbloomer_add_checkout_fee() {
WC()->cart->add_fee( __('Transactiekosten', 'woocommerce'), 0.29 );
}

There is a problem though with coupons. Our sponsors get five free tickets per year. I know how to do the settings for that, BUT they still have to pay that € 0,29
And that is not how we want it. For coupons it should be € 0,00

Is there a fix for this? Or should I do it different with the cost?

Thanks for your ideas!
Rob

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