You will have to change the table properties in the database. If you plan to enter the zip codes via the admin then you will have to change the properties of the field used to enter the data, it was not intended to enter long strings.

As for group discounts, yes the mod will work with them, but a lot depends on the sort orders for the Order Total modules as well as the setting for the group discount in the admin under Modules -> Order Total.

LadyHLG