just a minor clarification. even if the ticket has 0 days left, as long as it's within that 14 day from first use period (and there's been less than 10 days on it total), you can add to it.
if it's a 7 day ticket and you want to add 3 days, you pay the difference between a 7 day and 10 day ticket, even if there are 0 days left on it.