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function custom_payment_gateway_disable( $available_gateways ) {
    // Check if we are on the checkout page but not on the order-pay page
    if ( is_checkout() && !is_wc_endpoint_url( 'order-pay' ) ) {
        foreach ( $available_gateways as $gateway_key => $gateway ) {
            if ( $gateway_key != 'b2bking-invoice-gateway' ) {
                unset( $available_gateways[ $gateway_key ] );
            }
        }
    }

    // Check if we are on the order-pay page
    if ( is_wc_endpoint_url( 'order-pay' ) ) {
        if ( isset( $available_gateways['b2bking-invoice-gateway'] ) ) {
            unset( $available_gateways['b2bking-invoice-gateway'] );
        }
    }

    return $available_gateways;
}

add_filter( 'woocommerce_available_payment_gateways', 'custom_payment_gateway_disable' );