The point is that the OP now knows that she didn't pay for them, and whatever she does with them now, that does not involve taking them back and offering to pay for them, would make the OP a thief! Obviously, she can - and should - explain to them that she actually does still want the shorts, but in the correct size, and if they are at all reasonable, they will help her make the simple swap, whilst of course still paying for the shorts.
I am quite surprised that at least several of you Mumsnetters think it is all morally ok for her not to return the shorts, but to donate them to a charity shop instead! That is called taking the law into your own hands, which is not acceptable whatever depth of deception is at play.
Yes, I agree that not returning the shorts to the shop and/or offering to pay for them, is small fry when we consider all the crimes happening right now on this globe that we are occupying and destroying. However, would the PPs who think that the shorts are no big deal, feel the same way about someone who's little Granchild saw some nice shiny jewellery on their way to the baby change in a department store, and not knowing any better, dropped it into their Grandma's bag (yes, that happened to me, but luckily I was watching them, so I was able to put it straight back, while explaining gently to my DGC why they mustn't do something like that again), and poor Grandma finds it when she gets home, think that "oh the poor Grandma is going to be very embarrased, so she should just 'donate' it to the posh charity shop on the other side of town"?
Maybe a bit more background would be helpful at this point? No? Oh well, I will type it anyway, knowing that you have probably given up reading this now! 🙈
When Grandma saw the price tag on the fine chain of the unusual, but pretty necklace, she saw that it's price was reduced from £896 to £696 - which still made it a very expensive item in Grandma's eyes, therefore she started to feel a bit panicky, but then realised that the shop would still be open for another 20 minutes, so she rang the store and explained to the duty manager in the jewellery department, what had happened. They thanked her for her honesty (!), and asked her if she could bring it back in tomorrow, which of course she agreed to do. If you think that the Grandma did the correct thing this time, I am very interested to know where you draw the line between something being small enough that donating it to charity would be enough moral recompense, and something being expensive enough to own up about having unknowingly taken it home? I'm thinking that maybe the reasonable supermarket shorts would cost about £19, and in my little story, the necklace costs almost £700, so at what price point does it change from being morally ok to never let the supermarket know about the mistakingly stolen shorts, to it not being ok, to not return a stolen item? Would £55 be too much to shrug off, or should it be wrong if the unpaid for item cost £120 +?
As making that sort of arbitrary decision would be too difficult, isn't it better to just say "always return the unpaid for item"?