In our shop we often find items left or lost by customers.

Answer

It is Waajib to put up a notice in the shop about the lost item found. After one year if the owner has not come to claim the item, then it should be given to the poor or sold and the cash given to charity. However, even after having given the item to charity, if the owner appears, then the value of the item has to be given to the owner and you will then receive the thawaab of the Sadqah.