The following HTML code adds some links to easily send a Squarespace blog posts to the "Read Later" services Instapaper, Pocket and Readability. Add it to the "Post Blog Item Code Injection" and customise your CSS to suit.

"Read Later" links for Squarespace blogs

<dl class="readlater">
  <dt>Save:</dt>
  <dd><a href="http://www.instapaper.com/hello2?url={permalink}" target="_blank" title="Send to Instapaper">Instapaper</a></dd>
  <dd><a href="https://getpocket.com/save?url={permalink}" target="_blank" title="Send to Pocket">Pocket</a></dd>
  <dd><a href="http://www.readability.com/save?url={permalink}" target="_blank" alt="Readability" title="Send to Readability">Readability</a></dd>
</dl>

<!-- Optional CSS to use as a starting point. Add this to your site's custom CSS and remove the <style> tags. -->

<style>
dl.readlater dd, dl.readlater dt {display: inline; margin: 0 10px 0 0;}
dl.readlater dt {color: #464646;}
</style>