title: Firestore, why use "update" instead of "set merge"?
body: <pre><code>set with merge will update fields in the document or create it if it doesn't exists
  
  update will update fields but will fail if the document doesn't exist
  </code></pre>
  <p>Wouldn't it be much easier to always use set merges?</p>
  <p>Are the prices slightly different?</p>