There's plenty of surveys on this[1]. In this one driver wages and benefits make up around 40% of total cost per mile. That's why self-driving trucks are so lucrative.
This is a great resource, but there are a few things wrong:
1. Fuel prices were low in 2015 and have been rising since then, so 39% of marginal costs is likely a peak.
2. The chart you referenced is only for marginal costs, and doesn't include fixed costs associated with a trucking company. The actual percentage of driver pay to the overall operating cost of a trucking company will be lower once fixed costs are factored in.
3. Automated truck companies will also want to capture a fraction of that 30-40% that is paid to drivers.
So in the end, shipping costs will likely drop in the 10-20% range, which is substantial, but it isn't enough to be revolutionary.
My point wasn't to the value created by having automated trucks, but to counter the point that shipping costs would drop significantly.
Also, since shipping costs are a small percentage of the price of most consumer goods, everyday purchases will not be affected much either.
[1] PDF warning, see page 23 http://atri-online.org/wp-content/uploads/2016/10/ATRI-Opera...