Is $900 to install composite decking labor a fair price?

I framed my deck myself and hired a contractor to install the composite decking. I ordered all the material and just wanted someone with experience to install it. We agreed on a price of $900 for the labor. The job took him longer than he expected, and now he is asking for an extra 7 hours for "prep time" on the invoice, which I did not agree to. I'm seeking advice on whether this additional charge is reasonable. There were also a few mistakes made, such as some boards lifting and fascia screws being used on the outside picture frame boards on the stairs instead of the discussed sunken screws and plugs. While I was willing to accept these mistakes at the original $900 price, they become more irritating if the price increases significantly. His initial quote seemed low compared to others I received, which is why I double-checked and confirmed everything verbally and via text messages. I did not have a formal contract. The build was completed a long time ago, and he is now following up with the invoice. I appreciate the work and effort he put in, but it feels unsettling to be charged a price I didn't agree to. I'm hoping to work with him to come up with a mutually agreeable price.