From what I can see, these are low profit items, eg
one dozen ink pens. If a person is ordering office supplies, ordinarily s(he) wouldn't *only* purchase a box of ink pens. If you use one-click (a lot of people default to one-click), then Amazon is losing nearly (or all) of the $3.94 price on a box of ink pens for unnecessary shipping costs. The "add-on" restriction requires you to use the cart (as well as accumulating $25 in purchases) if you want the $3.94 price. Anything marked Prime is unrestricted; Add-on is "Prime, but..." It seems reasonable enough to me but that's JMO.