lucy24 - 9:28 pm on May 29, 2013 (gmt 0)
I assumed at first that the purchaser used a cached version of my page to make the order but he tells me he ordered the item on his mobile using Firefox.
How do you get from A to B? That is, how does "mobile + Firefox" translate to "can't have been cached"? Caching doesn't only happen within a physical browser. It can be done at several points in transit, including the user's ISP.
The important thing is that the server doesn't know a particular page should not be cached-- unless you tell it. And then you have to hope the ISP pays attention to this directive. (I trust the browser more than the ISP in this regard.)