Well, they lowered everything. Convenience aside, Amazon has created a marketplace is every sense of the word. Sellers compete against each other in the confines of the Amazon global marketplace, ship their stuff out of Amazon warehouses. This competition has lowered the price of many things and put pressure outside of Amazon as well, because even the same seller can’t sell as much outside Amazon, without Amazon logistics as they do within Amazon marketplace.
Amazon has also lowered seller margins, since these services cost money and only the most successful sellers gain enough through volume to make a profit.