A cracking wheeze

There is an evil genius in Peckham Library. Not among the patrons: the book stock is sound enough, but, were you researching a plan for world domination, you’d want more extensive reference shelves and perhaps quicker Wi-Fi.
 
No, the evil genius is on the staff. He or she was responsible for the offer that marked the library’s 10th anniversary: a free canvas bag for any user who withdrew ten books in one go.
 
Some readers may already be snorting at the spendthrift generosity of this. But Peckham Library’s anniversary is worth marking – it is, for better or worse, an icon of regeneration. And the offer is more cunning than it initially seems.
 
The effect of the free canvas bag is to make people – by ‘people’, in this context, I mean ‘me’ – borrow many more books than they usually would; more than they can get through in the four-week loan period. The total overdue fees on ten books in the borough of Southwark amount to £2 a day. You can renew online, of course, which has the handy side-effect, if you’re as disorganised as I am, of meaning that you loose track of when the books should go back, because the date stamped in the front no longer applies.
 
With eight of my canvas-bag books read, I have so far accumulated overdue fees of slightly more than £30. I am prepared to believe that I have been unusually stupid; but I doubt I’m uniquely stupid. If only the evil genius could arrange the accounts to reflect it, those free canvas bags might show a significant profit.