AFAIK German law doesn't care how my filled out and double-sealed vote gets to the election office. Although I also don't see a point of why I'd hand it to some party representative when I can just drop it in a mailbox (I guess it might make sense as a service for less mobile people - who also can authorize someone to go fetch the voting materials for them), and I'd guess people just going round and ask to collect vote envelopes would cause lots of suspicion - door-to-door canvassing isn't really a thing here.