Well, the people paid for the cels only, so the 
transaction is over and you have no obligation to 
them otherwise. Now, if you wanted to be super-
nice re-unite the cels & sketches, pitching in 
the $.60 or so for 1st class is reasonable. 
However, I don't think you're under any 
obligation to pay shipping, especially priority 
shipping since you could easily end up paying a 
lot of money for it.