somebody needs to write the script, maintain and debug it, handle customer questions related to the option, ... The fact that we can automate stuff doesn't make it free. Also, as any good consultant will tell you, the price you can charge your customer is not dependent on how much work something is for you but rather how much value the customer gains. Installing FF on a computer takes me like 15 minutes if I include everything - from asking the password, installing, migrating any bookmarks, cleaning the download etc. That amounts to an hourly rate of about 60 GBP, a reasonable price if you ask me.
Not entirely. It's more likely some human intervention is needed somewhere in the process to select the image/install containing Firefox. Although it's likely something like a checkbox, or maybe fysically selecting another disk - 16.25 seems a bit steep for that, yes.
It actually does require a chackbox - customer checks it when ordering ;)
Booting computer and manually installing OS/updates requires quite a bunch of [man] hours (given that they ship a bit more than few PCs a day when distributing a batch) so it is safe to assume they do not do that, but instead clone disks. it is most likely that master is kept up to date by a script.
And a script is not written (and not necessarily executed) by itself.
It could be argued that the price is not fair, but it's still an option; it's common practice to add such options, in this price range, when purchasing computers online.