Barbara Kahn | Source | Professor of Marketing at The Wharton School - University of Pennsylvania

We know from psychology that people like to segregate gains — in other words, take pleasure individually in each positive benefit. But [they like] to integrate costs — in other words, they rather take pain in one lump sum than to experience the painful pricks each time. If the shipping price is incorporated in the price of the good, and customers don’t have to think about that pain tax, they would definitely prefer it.