title: How to add an additional value to a Shopify checkout URL?
body: <p>My client's live Shopify setup has a feature where a URL goes to a sort of "quick checkout" that allows a customer to quickly purchase a single item. The way it works is, you first visit a URL like this:</p>
  
  <p><code>https://site1.com/cart/xxxxxxxxxxx:y</code></p>
  
  <p>and then you are immediately redirected to a URL like this:</p>
  
  <p><code>https://www.site2.us/aaaaaaaa/checkouts/bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb</code></p>
  
  <p>And then on that second URL you can fill out the checkout form to purchase a single item that is displayed there. Notice that the two URLs use a different domain name. The <code>x</code>'s are numbers, the <code>y</code> is a number. The <code>a</code>'s are numbers and the <code>b</code>'s are hexadecimal digits(0-9, a-f).</p>
  
  <p>Is this a common/documented URL flow for a Shopify single item checkout setup?</p>
  
  <p>What I need to do is add a value to the first URL so that it carries through to the second URL, eg:</p>
  
  <p><code>https://site1.com/cart/xxxxxxxxxxx:y/extra-val-here</code>
  <code>https://www.site2.us/aaaaaaaa/checkouts/bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb/extra-val-here</code></p>
  
  <p>And then if the second URL has the extra value I need to do something extra in the code.</p>