I had to return a single ticket to Ticketmaster for one of the Toronto shows – because I did it within 24 hours of the original purchase, I received a full refund.  After 24 hours, Ticketmaster refunds the value of the ticket (i.e. they hold on to the additional fees like the “convenience” charges).  I am not sure what happens if the tickets have already been mailed out or if they are in your possession already – I assume you would have to physically go to a Ticketmaster outlet to return them.  Also, I assume the return policy is the same as the US a it is in Canada, but I do not know that for sure.
 
Cheers,
Bruce