Adding custom fields to the checkout form is easy to do with just a couple of functions. The code below will allow you to add two additional fields, one for Alternate Phone and Company Short Description.

invoicing-add-custom-checkout-fields.php

/**
 * Adding a custom field to the Invoicing checkout form.
 *
 * Covers:
 * 
 * Adding the alternate phone & company short description fields to the checkout.
 * Making the alternate phone field mandatory.
 * Setting an error when the alternate phone field is empty.
 * Storing the alternate phone & company short description fields into the invoice payment meta.
 * Adding the alternate phone & company short description to the invoice view/print page.
 * Adding the alternate phone & company short description to the emails.
 */

// Display custom fields(alternate phone, company short description) under billing address section in checkout form.
function wpi_custom_display_checkout_billing_fields( $billing_details ) {
    $alternate_phone = isset( $billing_details['alternate_phone'] ) ? esc_attr( $billing_details['alternate_phone'] ) : '';
    $company_desc = isset( $billing_details['company_desc'] ) ? esc_attr( $billing_details['company_desc'] ) : '';

    ?>
    <p class="wpi-cart-field wpi-col2 wpi-colf">
        <label for="wpinv_alternate_phone" class="wpi-label"><?php esc_attr_e( 'Alternate Phone', 'invoicing' ); ?><span class="wpi-required">*</span></label>
        <span id="wpinv-wpinv_alternate_phone-wrap">
            <input name="wpinv_alternate_phone" id="wpinv_alternate_phone" class="wpi-input form-control" required="required" type="text" value="<?php echo $alternate_phone; ?>" />
        </span>
    </p>
    <p class="wpi-cart-field wpi-col wpi-colf wpi-coll">
        <label for="wpinv_company_desc" class="wpi-label"><?php esc_attr_e( 'Company short description', 'invoicing' ); ?></label>
        <span id="wpinv-wpinv_company_desc-wrap">
            <input name="wpinv_company_desc" id="wpinv_company_desc" class="wpi-input form-control" type="text" value="<?php echo $company_desc; ?>" />
        </span>
    </p>
    <?php
}
add_action( 'wpinv_checkout_billing_fields_last', 'wpi_custom_display_checkout_billing_fields', 10, 1 );

// Make custom field(alternate phone) mandatory field.
function wpi_custom_checkout_required_fields( $required_fields ) {
    $required_fields['alternate_phone'] = array(
        'error_id' => 'invalid_alternate_phone',
        'error_message' => __( 'Please enter a valid alternate phone number.', 'invoicing' )
    );

    return $required_fields;
}
add_filter( 'wpinv_checkout_required_fields', 'wpi_custom_checkout_required_fields' );

// Set error if custom field(alternate phone) is not valid. 
function wpi_custom_validate_checkout_fields( $valid_data, $post ) {
    if ( empty( $post['wpinv_alternate_phone'] ) ) {
        wpinv_set_error( 'invalid_alternate_phone', __( 'Please enter your alternate phone number.', 'invoicing' ) );
        return;
    }
}
add_action( 'wpinv_checkout_error_checks', 'wpi_custom_validate_checkout_fields', 10, 2 );

// Store custom fields values(alternate phone & company short description) in invoice meta.
function wpi_custom_store_custom_fields( $invoice, $saved ) {
    if ( empty( $invoice ) ) {
        return;
    }

    if ( isset( $_POST['wpinv_alternate_phone'] ) ) {
        $invoice->update_meta( '_wpi_custom_alternate_phone', sanitize_text_field( $_POST['wpinv_alternate_phone'] ) );
    }
    
    if ( isset( $_POST['wpinv_company_desc'] ) ) {
        $invoice->update_meta( '_wpi_custom_company_desc', sanitize_text_field( $_POST['wpinv_company_desc'] ) );
    }
}
add_filter( 'wpinv_invoice_save', 'wpi_custom_store_custom_fields', 10, 2 );

function wpi_custom_checkout_billing_user_info( $user_info, $invoice ) {
    if ( !empty( $invoice ) ) {
        $user_info['alternate_phone'] = $invoice->get_meta( '_wpi_custom_alternate_phone' );
        $user_info['company_desc'] = $invoice->get_meta( '_wpi_custom_company_desc' );
    }
    
    return $user_info;
}
add_action( 'wpinv_checkout_billing_details', 'wpi_custom_checkout_billing_user_info', 10, 2 );

// Display custom field (alternate phone) under user address. 
function wpinv_custom_display_field_in_user_address( $invoice ) {
    if ( !empty( $invoice ) ) {
        $alternate_phone = $invoice->get_meta( '_wpi_custom_alternate_phone' );
        
        if ( !empty( $alternate_phone ) ) {
        ?>
        <div class="alternate-phone"><?php echo wp_sprintf( __( 'Alternate phone: %s' ), esc_html( $alternate_phone) ); ?></div>
        <?php
        }
    }
}
add_action( 'wpinv_display_to_address_bottom', 'wpinv_custom_display_field_in_user_address', 10, 1 );

// Display custom field (company short description) below invoice details. 
function wpinv_custom_display_fields_on_invoice( $invoice ) {
    if ( !empty( $invoice ) ) {
        $description = $invoice->get_meta( '_wpi_custom_company_desc' );
        
        if ( !empty( $description ) ) {
        ?>
        <div class="row wpinv-company-desc-info">
            <strong><?php _e( 'About company:', 'invoicing' ); ?></strong>
            <p><?php echo $description; ?></p>
        </div>
        <?php
        }
    }
}
add_action( 'wpinv_invoice_print_after_top_content', 'wpinv_custom_display_fields_on_invoice', 9, 1 );

// Add custom field (alternate phone) in billing details in email that send to the user. 
function wpinv_custom_add_field_in_email_billing_details( $invoice ) {
    if ( !empty( $invoice ) ) {
        $alternate_phone = $invoice->get_meta( '_wpi_custom_alternate_phone' );
        
        if ( !empty( $alternate_phone ) ) {
        ?>
        <tr class="wpi-receipt-phone">
            <th class="text-left"><?php _e( 'Alternate phone', 'invoicing' ); ?></th>
            <td><?php echo esc_html( $alternate_phone ) ;?></td>
        </tr>
        <?php
        }
    }
}
add_action( 'wpinv_email_billing_fields_last', 'wpinv_custom_add_field_in_email_billing_details', 10, 1 );

// Add custom field (company short description) below billing details in email that send to the user. 
function wpinv_custom_add_field_below_email_billing_details( $invoice ) {
    if ( !empty( $invoice ) ) {
        $description = $invoice->get_meta( '_wpi_custom_company_desc' );
        
        if ( !empty( $description ) ) {
        ?>
        <div id="wpinv-email-billing">
            <h3 class="wpinv-address-t"><?php echo __( 'About Company', 'invoicing' ); ?></h3>
            <p><?php echo $description; ?></p>
        </div>
        <?php
        }
    }
}
add_action( 'wpinv_email_after_billing_details', 'wpinv_custom_add_field_below_email_billing_details', 10, 1 );