In her first year at Amazon.com, Beryl Tomay made a software code change for Amazon’s order-confirmation page that caused it to appear blank for customers for over an hour. Later on, she made a change to the database of the Kindle e-reader that prevented users from signing in or downloading anything. That error was so large that Jeff Bezos, Amazon’s founder, noticed and emailed to ask about the problem.

Today, Tomay is in charge of a crucial part of Amazon that is in its busiest period right now: She oversees businesses and technology at the company’s “last mile” delivery unit—the logistics business that gets packages through the final steps on their way to customers’ doorsteps. It’s a chokepoint that will help determine whether Amazon is successful during the make-or-break holiday season.