If you are going to be spending the money on a bike with Di2 or eTap, then I'd think the LBS will be a little more lenient in allowing you to do a decent test ride. Even if you can't ride a Di2 equipped bike, maybe they'll have the same model in a 105 or ultegra group you can try. Hopefully in different sizes.
Like many you might be in the overlap size of two frames. So a 10 mile ride might indicate some things to consider. Sore shoulders? Feel like your legs are flailing because the bigger size suddenly comes with a much longer crank? Feel like you slide forward in the saddle?
I recently did both a parking lot ride and then came back with my cycling shoes and clothes and did a 10 mile ride each on two different frame sizes. The frame size I picked based on the 10 mile ride was not the frame size I would have picked based on the parking lot ride.
Most issues of fit can be corrected somewhat as long as your frame is in the ballpark of what your size is. Some issues are cheap to fix, others not so cheap, but still they can be corrected.