Quibans 1 - Black Friday

Here is Quibans 1.
I projected this:

"Amazon said that at 9.10pm, Black Friday 2015 became its biggest sales day ever in the UK with more than six million items ordered."

Carry out some calculations to help you decide how many items were ordered in the whole day.
Why might your answer be an under-estimate?
Why might your answer be an over-estimate?

Save this part until after they have answered the earlier questions:

"The UK version of the website sold more than 7.4m items while other retailers had problems with web sales due to huge demand."

What do you think now?

A follow-up question might be: 

On average, how many items did they sell per second on Black Friday?

If you assume items are bought at the same rate throughout the day and use exactly 6,000,000 as the number of items sold by exactly 9.10pm then you get about 6.8 million items sold across the day.

Does this mean that more people shopped later in the evening?  Or was the "more than six million items" really 6.5 million?  (Making the assumption that if it were over 6.5 million then "nearly 7 million items" would have been used.)  In this case you get 7.37 million items over the whole day, which rounds to 7.4m.

Links to the stories from which the quotes are taken:
http://www.manchestereveningnews.co.uk/whats-on/shopping/web-retailers-break-records-black-10517322
http://www.bbc.co.uk/news/uk-34952739