Seachaid is indeed the root, but it’s a verb like siúil in that the root is broadened before conjugating, e.g. seachadaim, seachadann sé, seachadaimid, seachadtar.
That's more of a question for English as for Irish - "Food is delivered every night" usually means "delivered (as a parcel)", but there are circumstances where you might use "served" or "delivered" with regard to food (are meals on planes "served" or "delivered"?).
"seachaid" is used for delivering a letter or a parcel, "riar" for serving food.