How Do I Add a Custom Fee at WooCommerce Checkout?

How to Add a Fee With the WooCommerce Fees API

The woocommerce_cart_calculate_fees action runs during cart/checkout total calculation, and calling WC()->cart->add_fee() within it adds a named fee line to the order total, displayed clearly alongside the subtotal, shipping, and tax.

add_action( 'woocommerce_cart_calculate_fees', function( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
        return;
    }
    $cart->add_fee( __( 'Handling Fee', 'textdomain' ), 4.99 );
} );

Things Worth Getting Right

• The admin/AJAX check shown above is important — without it, the fee can be added incorrectly in admin-side calculations, causing confusing discrepancies.
• Percentage-based fees are calculated the same way, just computing the amount from $cart->get_subtotal() or the full cart total rather than using a fixed number.
• Conditional fees (only for orders below a minimum, or only for a specific payment method) need additional logic within the same hook to check the relevant condition before calling add_fee().
• Tax handling on the fee itself needs deciding — add_fee() accepts a taxable parameter, and getting this wrong can cause the fee to be taxed (or not) incorrectly relative to the store's actual tax obligations.
• Displaying the reason for the fee clearly to customers (a well-chosen fee name, or accompanying text) avoids confusion and support questions about an unexpected line item.

Implementing the Fee Properly

1. Use the woocommerce_cart_calculate_fees hook with the admin/AJAX guard included.
2. Decide the fee's tax treatment correctly for the store's specific tax obligations.
3. Test with different cart combinations, including any conditional logic, to confirm the fee applies exactly when intended.

Need a custom or conditional fee built and tested properly? See custom WooCommerce development.