$filename= '/path/customers.csv';
if (file_exists($filename)) {
    $file = fopen($filename, 'r');
    while (($line = fgetcsv($file)) !== FALSE) {
        $customerid = $line[0];
        $customer_name = $line[1];
        // this is where I am needing assistance, or guidance if I am way off base
        // I need to query each "estimates" post and find it's "customerid" field value,
        // if it matches $customerid - $line[0] from the CSV, I want to update the "estimates"
        // custom post type's "customer_name" field with the value of $customer_name - $line[1]
    }
}