As a precursor, while there's a finite definition of what an hour is, that goes out the window when you're hiring developers. Some agencies charge for account management and project management time, others do not. A senior Magento developer may be able to get something done in 10 minutes that takes a less experienced developer an hour. While this is all normal, it is worth noting.
+/- 10 hours for site review is reasonable. In essence, if your site is buggy or has issues, you're going to, to at least some extent, hold your active developers responsible. Additionally, if the site, as it stands, is a train wreck (not that I'm suggesting that's the case here but in general), it's something you should know before you dump more money into it. They should audit to make sure everything is healthy and follows basic standards. If not, they're not heading into a healthy relationship with you. 16 hours is not particularly unusual for that process.
4 hours to produce a scoping doc also sounds pretty reasonable, although many agencies that I've worked with would not charge for that time. Then again, those agencies typically use a blended rate of $100 to $200 per hour in order to have to track account and project management time the way that they do production time, so there wouldn't be a net savings.
When it comes to individual tasks, if you're being charged for development in a local environment, testing and quality assurance, syncing to your dev server / staging server / live server, etc. time can add up quickly. Proper testing often involves not only checking for errors but checking for things like mobile and desktop views, and cross-browser testing for different types of web browsers.
I don't think that you're specifically being taken for a ride, but I wouldn't be surprised if you're paying for account management, project management, and QA time, especially at that low hourly rate.
My recommendation would be to get some competing quotes and see if there's an agency that better aligns with your expectations. Whether or not this agency is fairly priced is subjective, but you're certainly free to find better options. There's a wide spectrum out there.
