Dear Tesco Self-scan Machine,
people who know me will tell you that I am a person of modest ambitions, who does not expect a lot from life, so it may surprise you to learn that I am writing to you to suggest that you raise your expectations. Specifically, you seem to have a very narrow mind with regards to things that might be expected to appear in your bagging area. Bags, for instance, are something that you should probably start to expect. The clue, you see, is in the name: bagging area. In order to bag things, one must sometimes put bags in the bagging area; and while I appreciate that in general you can be confused when non-bought items appear in there, I think that saying my Bag for Life is 'unexpected' is a little extreme. 'Undesirable' might be a better way to express yourself.
Furthermore, I am concerned that you run the risk of being "the machine that cried wolf" when you get all over-excited by the presence of a plastic bag. If you do all your screaming at that, what options are left when I lob a horse in there? Eh? Or a cello? I just think you need to make sure you leave yourself a bit of leeway, because you don't know what a day will bring forth.
That is all.
With best regards,