I think, possibly, where I grew up (Limerick) the usage may have come from the practice of giving a small child a note wrapped round the money for the purchase, It was usually used for single items or a small number, usually to a nearby corner shop, often for stuff forgotten during "real shopping". sometimes a neigbour would ask a passing child to "go on a message" and the tradition was ( if it was not for family) then a "reward" of a penny (or a few pennies) would be given.