Woocommerce - Second T&C checkbox doesn't prevent checkout when left unchecked

I have a special checkbox I need placed only if the customer purchases one of two specific products. Code below looks for these product_ids, and if found, displays the additional Terms and Conditions box next to the regular T&C checkbox. While this part is working, the checkout still completes even if the box is left unchecked, so I'm guessing the problem is within the aym_not_approved_amb_terms() function, but I can't seem to figure out what that problem is.

add_action( 'woocommerce_after_checkout_form', 'aym_add_ambassador_terms_box' );
function aym_add_ambassador_terms_box() {
    // set product IDs:
    $product_ids = array( 17558, 17563 );
    $bool = false;
    foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
        $item = $cart_item['data'];
        if ( in_array( $item->id, $product_ids ) )
            $bool = true;
    }
    // If product IDs detected
    if ($bool)
        // add ambassador terms checkbox
            add_action('woocommerce_review_order_after_order_total', 'aym_add_checkout_tickbox' );

            function aym_add_checkout_tickbox() {
        echo '<script>console.log("checkbox")</script>';

                echo '<p class="form-row-wide terms">';
                echo '<input type="checkbox" class="input-checkbox" name="amb-terms-check" id="amb-terms-check" />';
                echo '<label for="amb-terms-check" class="checkbox">I accept Ambassador terms and Conditions</label>';
                echo '</p>';

            }

            // Show notice if customer does not tick

            add_action('woocommerce_checkout_process', 'aym_not_approved_amb_terms');

            function aym_not_approved_amb_terms() {
                if ( ! $_POST['amb-terms-check'] )
                    wc_add_notice( __( 'Please agree to the Ambassador Terms and Conditions' ), 'error' );
            }

}

Expected behavior: if the box is left unchecked, page scrolls back up to the error message area and displays the message 'Please agree to the Ambassador Terms and Conditions'. Maybe I'm not using the correct hook for the aym_not_approved_amb_terms function...?