I agree with this: Erase and reinstall and then test for a few days.
If that doesn't work, and you have the time before your return period ends, then use
Apple Configurator from a second Mac to reset the firmware. I've heard of people (namely Marco Arment from Accidental Tech Podcast) having "invisible" hardware issues whereby using Apple Configurator fixed it.
Anecdotally, I get 10-hours of mixed usage with Safari (tons of tabs, including some YouTube) and other productivity apps; brightness setting is 100%.
I will caution that you should first read Activity Monitor and see what is using your CPU, to make sure it isn't a rogue app or background process. I'm in the habit of daily restarts because I've had apps bug out and use too much CPU and/or memory. Long battery life requires apps be mostly idle (as in, not use CPU other than for a split second, called 'burst processing'), so it only takes one app using the CPU for extended timeframes to ruin your battery-life-expectations. When you find an app doing that, delete and use a more optimized alternative (eg. Chrome using too much resource? Start using Safari).
Note that some people will naturally have workflows that use the CPU all the time and then wonder why their battery life isn't 16 hours—know that you won't get anything near that if you're using apps and workflows that often require CPU processing. You will only get 10-13 hours of battery life from using apps that are mostly reading (eg. Safari browsing) or streaming H.265 video, because the CPU is idle while you're reading or watching/listening to optimized media. Also, 16 hours isn't realistic for mixed use so lower expectations because Apple marketing numbers are creating false hope.
EDIT: If you haven't already, go to System Settings → Battery, then put Low Power Mode on "Only on Battery." What this does is prioritize most thread requests to use the efficiency cores, thus extending battery life. You shouldn't notice a speed difference for the most part so it's a great setting to turn on.