It is commonly heard in the business world to under-promise and over-deliver. In this way a business can impress a customer or client. How can this apply to the professional photography business? One simple way is to just give the client extra photos than stated in the package they bought. Which photos to give as bonuses? Well, they will of course not be any of the best of the best because those will have already been included in the initial product to the client.
One way to know is to listen to your client's feedback once you delivered the promised number of photos. If they say they like shots of one particular person a lot (often a child) or like one certain style of shot or if they say they really like the black and white versions, then any of those can be your clue on which photos to include as bonuses.
A recent client said she especially liked the photos of her daughter. So I went through my remaining unedited 3-star and even 2-star photos from our portrait session to see if there was anything I thought still good enough to put into the hands of a client. I found the above shot and made it work to my satisfaction by applying a sepia landscape filter to it in Nik Silver Efex Pro.
The result? The above image was included in the prints she later ordered, the direct business benefit to the photographer. Even more important, I was able to provide the client with another image of her daughter that will brighten a desk or shelf in her home. Thus, satisfaction for the client and satisfaction for the photographer knowing his/her work provided some real world benefit.