Well, the gist of it is that it's not that hard technically, but could be more difficult to implement organisationally. Production load-testing is something that needs to be agreed with various teams/people across the organisation (easier if you're a small startup), think ops, marketing, analytics etc.

The basic thing is to make sure you can separate real and synthetic requests (with a special header for example). This will allow you to mock/no-op certain operations like attempting to charge the user's card or reducing the quantity of stock you have. It'll also allow you to remove synthetic requests from your analytics data, so that marketing does not get excited by the sudden influx of new users. If you have user accounts on your system, make all fake users register with @somenonexistentdomain.com so you can filter for that too etc.

Obviously start slow and ramp up over time as you iron out issues.

JustEat.co.uk run daily load-tests in production at +20-25% of their peak traffic. As in: extra 20% simulated load during their peak hours, which happen to be between 6-9pm every day. They process a very respectable number of real-money transactions every second, a number that a lot of ecommerce sites would be very happy with. (Source: a presentation at ScaleSummit in London this year)

Feel free to @message me if you want to talk more about this.