That number has been offered for a long time, so it seems to be based on data that was collected years ago. Even then, the actual effect probably varied a lot from one category to another. Nevertheless, there is plenty of research which shows that buyers do favor items with "free" shipping.
Unfortunately, for items over a pound or two, the cost of shipping varies significantly depending on distance (unless the Priority Mail flat rate boxes are your best option, usually only for heavy items). This makes it difficult to set a fair price for your item when the cost of shipping is included in your asking price. Setting a flat price for shipping (including free) inevitably will overcharge some buyers and undercharge others.
The exception to the above paragraph would be Media Mail, which has a price that doesn't vary by distance. Sellers who ship Media Mail can easily add the shipping cost to their asking price though this does make it impossible to offer combined-shipping discounts for multiple purchases.
