"I order four kilograms of tomatoes."
Translation:Mi mendas kvar kilogramojn da tomatoj.
The word "of" is really the key. "Of" is the beginning of the prepositional phrase that describes the object of the sentence.
Think of the phrase: "I learned three words of Esperanto." Yes, you learned Esperanto; but in this sentence, you specifically learned "three words," and these words just so happened to be Esperanto. Coming back around, now we have the phrase: "I order four kilograms of tomatoes." Yes, you order tomatoes; but in this sentence, you specifically order "four kilograms," and these kilograms just so happened to be tomatoes.
I hope that makes a bit more sense
Yes. and when "I am ordering," the order has no defined end in time. I have started to order and I am continuing to do it, for a maybe a few more moments (until the waiter jots everything down), or maybe continuously, for a few more months (of consecutive shipments). But when "I order", it is a discrete event--or many discrete events that each have a clear start and end, instead of continuing throughout. Hope that's not confusing, since I somewhat flipped your explanation around.
da is for quantities and
de is for possession.
glaso da vino - a glass of wine (quantity)
taso da teo - a cup of tea (quantity)
la patrino de la knabo - the mother of the boy /the boy's mother (possession)
la koloro de la lakto - the color of the milk / the milk's color (possession)