显示获取的基本货币，然后转换为USD值并显示在自定义元框上

I have fetched the value of _order_total_base_currency (from Aelia Currency Switcher) which is already converting in AUD. Having this value I want it converted into USD and display on my custom meta-box.

The code I have right now is doing just that, but by having a fixed multiplier of 0.79 (since 1 AUD = 0.79 USD).

What I like to do however is replace the "0.79" to a variable value that is an updated exchange rate perhaps coming from Google API?

Here is my code:

add_action( 'add_meta_boxes', 'cdmb_add_meta_box');
function cdmb_add_meta_box( $post_id ) {

    add_meta_box(
        'woocommerce-order-my-custom',
        __('USD Currency display','wc-usd-display'),
        'cdmb_display_meta_box',
        'shop_order',
        'side',
        'core'
    );
}

// The metabox content
function cdmb_display_meta_box() {
    // Get
    global $post;

        $total_usd = (get_post_meta( $post->ID, '_order_total_base_currency', true )) * 0.79;
        $total_usd .= get_post_meta( $post->ID, '_order_total_base_currency', true );
   ?>    
     <table id="wc_cdmb_display_meta_box">

            <tr>
            <th><strong><?php esc_html_e( 'Order Total Base Currency', 'wc-usd-display' ) ?> : </strong></th>
            <td><?php echo esc_html( ( empty( $total_usd ) ) ? __( 'N/A', 'wc-usd-display' ) : number_format(floatval($total_usd), 2) ); ?></td>
            </tr>

    </table>
<?php
}

The third party plugin Aelia Currency Switcher is outside of this. I already got what I want from it which is the value of _order_total_base_currency. Use the value as source to convert it to USD.

I'll appreciate all your help.