If you host your site on WP Engine, you may encounter some issues with empty carts when visiting the checkout page. This is due to aggressive page caching. Page caching has both pros and cons, however, when it comes to online ordering- it prevents items from getting added to the cart and other issues. However, it can be easily resolved by requesting that WP Engine set up a special cache exception for your checkout page.
To do this, simply submit a support ticket to WP Engine via their support portal or use their Live Chat and ask them to exclude your checkout page from cache.
Your checkout page is the one that has the [moo_checkout] shortcode on it, and is saved in Pages (the default page is www.yousite.com/checkout):
To make things really easy for you, feel free to copy and paste this sample ticket:
Hi!
We are having some issues with page caching.
Could you please add the following URL to the exception list?
http://YOURSITE.com/{put your checkout page URL here}
http://YOURSITE.com/{put your cart page URL here}
Thanks!