Hi,

I think you should post to the webpage the 
sketches and have people send you scans of their 
cels to prove that the sketch belongs with their 
cel, and give a time frame (i.e. one month) for 
claiming the sketches to the appropriate owners.  
During that time period, I think you should suck 
up the shipping costs.  After all it was your 
mistake for not sending them in the first place.  
After the time period is up, you can sell the 
sketches or keep them or continue for looking for 
owners.

I recently had a similar situation.  Sketches and 
cels got mixed up and I sent out some cels w/o 
their sketches.  When I realized the mistake I 
sent out the sketches and my cost (since it was 
my mistake).  I had a lot of happy and surprised 
people.  Several have been repeat buyers.

I think paying for the shipping is just good PR.  
I don't think I'd be alone in being slightly 
upset to have to pay for someone else's mistake.