Collection / German Empire / Others / Postage Due

Foreign postcard from 03.09.1918 (2. Period) franked with one stamp a 7½ Pfennig (=7½ Pfennig);
correct postage: 10 Pfennig
postage due: 2½ Pfennig = 7½ Cent

Foreign cover (21-40 g) from 23.08.1923 (15. Period) franked with 6 stamps a 500 Mark (=3 000 Mark);
correct postage: 4 500 Mark
postage due: 1 500 Mark = 25 Danish Öre

Inland cover (21-100 g) from 05.11.1923 (23. Period) franked with 5 stamps a 200 Million Mark (=1 Billion Mark); correct postage: 1,4 Billion Mark
correct postage due: 600 Million Mark
collected postage due: 8 Billion Mark ??