COMMODA´TUM is one of those obligations which are contracted re

related words?

incommode, v. = to inconvenience, impose an uncomfortable obligation
commodity, n. = that which can be contracted for
accommodate, v. = make it easy or comfortable, do what's wanted

Seems to have some relation ot the Japanese "on," though that seems to have more the flavor of an obligation of honor rather than goods or services.

By extension - ? distantly related is
commode, n. = that article of bathroom furniture on which we sit on and qhich receives our waste products ?