Its going to entirely depend on the segment, Wal-Mart gets 55% of revenue from groceries apparently (I took some time to look at what fraction of groceries would be of overall retail sales, but couldn't find it). So I think a reasonable assumption is that a large chunk of retail sales are groceries. Another large chunk is going to be automobiles, which in some metrics get excluded; which are another large part of retail sales.
If you keep chopping away big parts that aren't conducive to e-commerce; the 6.40% is going to be a pretty big percentage. Small specialty stores are probably hit especially hard, to the point of being crushed; going out of business and making the e-commerce players a larger percentage of the market and thus more efficient, driving even more e-commerce in a positive feedback loop.