This is kind of tough, but here's how I would view
it. 

The cels were never mentioned in the sale, so you
probably got a better price on them. I'd bet that
the ending price would have been a bit higher if
it had been known that cels were also included.
(The DS fee would then have been higher too.) In a
sense, you received a bunch of freebie cels from
the YJ seller.

Since you spent the money anyways, thinking that
they were only sketches, the cels are really a bonus. 

True, the extra weight would have cost more. Also
true, that the DS probably should have noted that
there were cels included in the package. Even so,
 you came out with a good deal as it stands now.
The $13 non-cel fee (plus extra shipping weight)
still comes out to around $1-2 per cel, which is
not bad.

Customer service-wise, the DS could have been a
little smoother/quicker at working this out with
you. Still, it seems like a lot of work (for the
both of you) over a transaction were you came out
ahead on anyways. If anyone came out with less
than they should have, it was the YJ seller.

Please don't take this the wrong way. This is just
how I would view the situation. I think the issue
of whether the transaction qualifies as a cel sale
or not; and the bonus of extra/unexpected items
kind of cancel out $-wise.