I think I would look at it as, "Is the set
worth $120 to me?  Would I have paid that amount
even if it had been available in the USA?"  If
I were satisfied with the set, and the amount I
spent, I would probably just drop the issue over
$13.  

However, I'm also very picky about customer
service... Can you prove (through pictures or
scans or whatever) that the cels and the items
auctioned are actually the same items?  If so,
I can't understand why $13 would be such a big
deal to the service, and I would think they
would refund it with an explanation along the
lines of "We're a large business, handling many
customers, and it's nearly impossible to check
every order against the item number and also
against the billing."  After all, if the shipping
department didn't have some faith in the billing
department, how would anything get done?  I
would think they would refund the overcharged
amount on principle alone... however, if it's
turned into a huge effort on your part to make
them correct their mistake, I would probably
drop it and simply think twice about using them
next time.  What if it were a more significant
amount, such as $50 or $100?

(And for the record, I have no idea which deputy
you use, as I haven't even seen anything I've
wanted on YJ in months and don't tend to keep
up with "this week's sale".  ^.~)

Good luck!
Kim