Darin is right in my opinion: "Then again Germans are notoriously redundant in their phrasing."
I don't quite understand why Sebastian is asking what else it could mean. First full sentence: "Innerhalb von 2 Arbeitstagen nach Eingang bei Ihnen, müssen
alle Bestellungen schriftlich bestätigt werden."
If you need to confirm in writing that you accept the order, it's...to confirm it. Prior to this AB, it was just a request (unless the two workdays were up); now, there are two parties to the contract.
It is definitely not Auslieferungsbestätigung. The only other thing I could think of will knock you both off your feet because of its inherent redundancy: Auftragsbestellung.
For example:
https://stalderag.ch/de/zusatzleistung/auftragsbestellungCould be because confirmation is implied (or deemed given) after two days. What it is exactly, however, should be clear from the full context, I think.
Best wishes