Etsy API - update inventory

Question

I am using PHP and trying update an Etsy listing that has variations. My code is as follows:

$this->my_etsy->getAccessToken(site_url('inventory/etsy/update_price'));

    $api_url = 'https://openapi.etsy.com/v2/listings/<my_listing_id>/inventory';    
    $listing_data = array(
        'listing_id' => <my_listing_id>
    );
    $success = $this->my_etsy->client->CallAPI($api_url, 'GET', $listing_data, array('FailOnAccessError'=>true), $response);    
    //print_r($response);

    $new_data = json_decode(json_encode($response->results), true);
    $sub_data = [];

    foreach($new_data['products'] as $key=>$val){
        foreach($new_data['products'][$key]['offerings'] as $k=>$v){
            $price = ceil($new_data['products'][$key]['offerings'][$k]['price']['amount']*.8);
            $sub_data['products'][$key]['offerings'][$k]['price']['amount'] = $price;
        }
    }
    //print_r($sub_data);

    $arr = array(
            'products' => json_encode($sub_data),
            'price_on_property' => 513,
            'quantity_on_property' => '',
            'sku_on_property' => ''
        );
    $api_url = 'https://openapi.etsy.com/v2/listings/<my_listing_id>/inventory';
    $success = $this->my_etsy->client->CallAPI($api_url, 'PUT', $sub_data, array('FailOnAccessError'=>true), $response);
    print_r($response);

I've tried a bunch of different ways to pass the data and nothing works. The error that I am currently getting is:

[Server_Error] =>

Any ideas as to what I'm doing wrong?