I do miraculously seem to have 'taught' my 21 month old about paying in shops... not that it was a conscious effort. Just she got to the point while doing our grocery shop of spotting things she fancied. And I decided that if she spotted something that it was sensible for her to have (e.g. babybel cheeses; humzingers) then I would say 'yes, but you can't eat it until we've paid') and she would clutch the allowed item and as soon as we'd paid I would let her tuck in.
A few weeks ago, days after we breastfed for the last time ( ) she said 'boob, yilk' and I had to say 'sorry, it's all gone'... to which she replied 'shop, money, pay', presumably thinking we could pop down the Total and fill me up...
And she got fed up with me trying to choose a present in mothercare yesterday and started shouting 'pay' very loudly.
With all this in mind I think if she 'stole' something, going back in and demonstrating the act of paying for the item before leaving with it could possibly be a more positive way of showing her what you're supposed to do.