Where does one obtain such a thing?

For context: DHL was supposed to pickup a parcel and return it back to the merchant. The pickup apparently failed, because the pickup location (an office) wasn't open at the time the courier arrived.

This is purely within Germany, so I've not had any luck asking for help from the DHL International unit.

When I call DHL to reschedule the pickup, they keep asking me for a Warenkorb(?) number. My German isn't great, but that's what the word sounds like.

When I tell the rep I don't know what this is, they sound frustrated and pretty much hang up on me after. I'm incredibly confused.

The packet has no return Etikett; it only has its outbound Sendungsnr.

DHL had made a mistake delivering the package; the package had actually already been declined for delivery, via DHL's website, immediately after it was dispatched.

When I realized the postman's mistake I called DHL and they arranged a return pickup for free, and sent me an email with the Abholauftrag ID.

However that's the pickup that failed. Unfortunately the package also wasn't addressed to my name; I had ordered it to a package-forwarding company which allows only its company name to be used as the address.

DHL now refuses to reschedule the pickup until I can give them a Warenkorb Nr. Unfortunately I have on hand only the original Sendungsnummer, and of course the Abholauftrag ID.

It's possible that I misheard the term, but when I switched to the English-speaking department of German DHL they also used the same word. When I asked how to get this, the rep claimed that I should have one.

Anyone know what this is?