At the Zollamt you'll get to enjoy standing in at least 3 separate and consecutive lines. Only to pay import taxes. Something that could have been done faster, cheaper and more conveniently by sending an invoice.
This is how it works if the seller fills out CN22 properly and the customs officials believe the declared contents/value.
I have several times received packages with duties that I paid on delivery. No need to travel to the Zollamt.
I have also been to the Zollamt when they suspect the package contains something else than what is on the shipping manifest. They're always quite giddy to catch you doing something not allowed and always end up terribly disappointed and grumpy when it's exactly as described and then you just pay VAT.
Digital signatures by themselves only provide non repudiation of the sender non the receiver. When in court, you as a sender will have to prove that the receiver actually received the data. § 371 ZPO [1]
When sending a Mail per Einschreiben, the receiver instead has to prove that he didnt in fact get the message. Why that also applies to FAX blows my mind though.
AS4 and AS2 respectively solve this in a way, because the receipt is part of the protocol and successful transmission. From what I understand(I might be wrong) its traditionally used for EDI in supply chain systems [2]. Drummond certifies that your product supports the standard correctly some people make a lot of money and you're good to go(it's XML and SOAP though). Apparently the EU is looking at using it.
Here's where the german bureaucracy ruins everything though. The law stated above mandates from what I understand(again IANAL) that you use DE-Mail for communication to flip the procedure stated above. DE-Mail requires the use of a trust center, a third party that receives the message and forwards it. The only other way to flip the evidence case is by using your own contract, which german lawyers avoid, because they might be liable in case something goes wrong.
Fax has a built-in transmission protocol and the receiver confirms that the transmission succeeded.