MySQL supports transactions. This allows software developers to create application which preserve data integrity even in the event of (software or hardware) failures.
The work to do is to use transactions as provided in MySQL to protect all collections of queries which should be treated as being atomic in nature.
Another important feature of MySQL is stored procedures. These procedures exist in the database and, when called from a PHP application, can greatly increase performance. Furthermore, stored procedures allow multiple applications, written in PHP or not, to behave in a consistent manner.
The work to do is simplify the checkout of cart items using stored procedures.