I recently bought a pair of Costa Del Mar Hammerhead sunglasses and I love the lenses but the part that goes on your ear can get pretty uncomfortable if worn for a while.  

Is there some sort of cushion that can be put there to make them more comfortable?

I just use a neoprene strap.  It serves two purposes, one keeps me from losing my glasses in the water, and it adds a little bit of padding.

Although,if they are that tight that they are causing discomfort, they may be too small.  There are a couple of styles of Costas that I like,but they just don't make them in a large enough frame for my big melon.  You may need to take them back and get a different frame that is better sized for you.

They are actually pleanty big, if anything they are to big which i like cause the lenses are the biggest they make.