They're definitely related to cels. But in a way, 
you could also say that about cel books.

It seems like nitpicking to me, but if Rinkya is 
having a sale on shipping (or such) for cels and 
they only named *cels* as the item getting the 
discount, then you're stuck. Although related, 
sketches aren't actually cels. Me, I would have 
asked first to find out if the discount included 
sketches. (Or simply have been prepared for them 
to not be included.)

Many Sharp Smiles,
--Drac