We have one product that makes up the vast majority of buyer returns, very annoying as there are very clear dimensions and what it will fit and won't fit. 

Sadly, many buyers do not read the item description, choosing to look at the photographs instead. We added £1.00 to the product to cover the returns which probably run at 1 in every 10 purchased. 

Our postage cost to the buyer represents 30% of the product price, its quite a hit for a return.