The legal charge would be larceny (which could be a felony or misdemeanor depending on the amount of money). To make out a charge for larceny, the prosecutor would have to show that you had an Intent To take the envelope from the person without any intentions of giving it back (i.e. stealing the envelope). Also, because you were an employee of the store, the could also charge you with fraudulent conversion or embezzlement, but this is unlikely given that it was not the store's money but a customers.