Upsert replaces stored listing-default cleaningFee only when cleaningFee is sent; omitting it keeps the stored default. This PUT cannot clear only the listing default while keeping overrides — use DELETE on the listing to remove the whole document. channelConfigurations is required: send [] to store no overrides (clears existing). Omitting channelConfigurations is 400. An override entry may omit cleaningFee (channel only); GET by channel then falls back to the listing default. Do not send _id or lastUpdated on write; they are server-owned. When shouldUpdateSubUnits is true, child listings are overwritten with the same payload (not a merge of each child's previous settings).