Perhaps you could contact the people you sold 
them to and offer to split the cost of shipping 
the sketches to them? That way you're not out 
that much money, but you still do sort of "pay" 
for your mistake.

Really, though, I don't think paying shipping to 
get a lost douga is all that difficult--I'd 
probably do that if I had ever bought something 
and been told they forgot the sketch.