I agree with Sco08y on the text field for the invoice number (even though it wasn't your original question)
General rule (as it was taught to me by one of my professors many years ago) was that if you did not need to perform any sort of computation on the datafield, then it should be a text field vice a number field.
One of the other advantages besides distinguishing leading zeroes is if, for whatever reason, the spec for the invoice numbers should change and you would be required to use non-numeric data.. i.e. A12346Z. By having it as a text field right from the getgo, you have the flexibility to change the input criteria with no real impact on the structure of your database.