Custom Checkout is a convenient way for the customers to purchase something online. One-click Upsell is a Shopify app that helps you increase your conversion rates by offering an upsell offer during checkout. And provides custom buttons and more to your Shopify store's checkout page. The most important thing about any checkout process is its simplicity. This means that it should be easy for the customer to complete the purchase and go through all the steps in order to buy something.