There may be couple of factors.
1) Local taxation: The state taxes a lot to its citizens and business.
2) Labor costs: The cost of labor is one of the (if not the) highest in the world. In manufacturing industry, very high productivity of German workers compensate for high labor costs. It is likely that efficiency model of manufacturing industry; do not readily fit into a logistics company. And consequence is rise in
unit delivery cost. On the contrary, very backbone of logistic industry is one of the (if not the) best in world. German infrastructure, notably its autobahns, airports and rail network, should more than compensate for high labor costs.
3) Regulated Market: Upon posing a question of volume discount / loyalty program to Hugendubel saleswoman here in Frankfurt, I got a reply, there is German regulation that forbids book being sold under certain amount and book prices are set by an authority.