Hired a contractor to do a job?

Install skylights was the job. It was done on the outside and looks good. The problem is on the inside! The skylight ends six inches above the drywall. Rain does not come through but it looks bad. The boss came to look at it and agreed it looked bad but said that roofing is his field not skylights. He didn't mention that on the phone when I hired him. Is it normal to hire a contractor to do one job and have to hire another to make it look nice? The bill for the job is $1800.00. I feel scammed. Is that what this is or is this typical?