I had a slightly similar thing happen. I bought DD a pair of shoes in a well known dept store, they were reduced from £35 to £5. I was SO pleased. She was only 13months old at the time. They put them on her to try them on and she got stroppy so we ( assistant and myself) left them on her. They gave me the box to go and pay with when I was ready. I did some more shopping and totally forgot they were in the pram hood.
Walked out, shopped in waitrose, walked home, a fairly long walk ( for me) it was cold too. realised as I unpacked the pram.
many friends told me to just keep them, it was only a £5, that they make enough profit etc... i felt SO bad, I took the box back about 2 weeks later ( next time I could go). I felt SO guilty that whole time, I really did.
Went to the shoe dept. explained everything. She scanned the label, and said " these are discontinued and out of stock madam" I said well these might be the last pair. She said, I can't sell them to you, as we don't sell them anymore.
I was very confused and said, but I have them already, I just forgot to pay, DD has been wearing them for 2 weeks. She got her manager. The manager understood what had happened and said, we can't sell them to you madam, they show as out of stock and discontinued. I told her they'd been reduced in the sale and were £5.
She said there was no way to verify it now and no way to take the payment. She smiled and said perhaps you should leave the shoe department until your daughters feet grow again, get her measured in 6-8 weeks. And that was that.
Free shoes from a well known dept store. now, would this be called karma, for being honest? or something else?
OP: just take it back next time and explain. You will feel better for it.