Enough to pull orders, not enough to push stock
The Zalando connection is set up with three required values: a client ID, a client secret and your merchant identifier (bpid). Those three get your orders flowing. But to push stock and prices you need one more: the sales channel ID. On Zalando, stock and price updates happen per channel, so without this identifier nothing can be pushed.
On the setup screen this field looks optional, and that is what makes it confusing: technically it can be left empty, but if you do the connection works only halfway. Orders arrive; stock does not go out. We spell it out here because that halfway state does not announce itself.