Proof?

A patron just asked me to get her a recipt for a book fine that didn't exist.  One that she paid while I was standing there, seconds after informing her of said fine. 

Normally asking for a recipt is not an odd request, except that the type of fine she paid never registers in our computer system, and thus, normally, a recipt is never printed.  (It should also be noted that people who ask for recipts are stubborn and distrustful of the library in general, as they seem to be of one mind that your Friendly Public Library is in fact trying to rob you blind by saying you have fines when you really don't, and/or not marking that you paid them after you did.)  The particular charge that we were now wrangling with is a courtesy charge that we apply to certain kinds of book requests, but our circulation software doesn't know this, and we normally don't input such a charge into the system, as 99% of the time it is paid in full the moment we ask the patron to pay and a recipt is never waranted.  She did in fact pay for it the moment I asked her for the spare change it amounted to, but in asking for a recipt, she was asking for proof of payment that the computer didn't even know she owed.  So, we spend the next 30 seconds in amicable yet strained silence as I manually entered a fine onto her account, then went right back in and told the computer she had paid the amount in question (which, as it happens, is only 50 cents).  I could have explained to her that what she was asking for was ridiculous and waste of time, paper, and energy, but as noted above, people who ask for recipts should in general not be messed with.

She now thinks that she has some kind of solid proof of payment, and I think she is a moron.  I picture her gloating over her poorly-earned recipt, imagining with relish the moment where she might wisk the slip of paper out of her wallet and wave it under the librarian's nose who next tells her she has a fine.  "A ha!" she will shout, "I have a recipt, and proof that I have already paid!" when in fact she hasn't, the fine now being asked of her is legitimate, and all she has is evidence that I can successfully put amounts into and out of our circulation computer.

Such is life.