Re: "How can different industries have different rules?"
Besides being dead for anything "Amazon sized" for a decade or so the "Briefmonopol" has absolutely NOTHING to do with this. BM was a rules than only "The Gilb" aka Germans state owned Bundespost could deliver any kind of mail. That has long stopped for packages of any size. DHL, Her(p|m)es, Uuups etc. all deliver.
What rules of payment (if any) apply depends on what "Unternehmerverband" (businessmen's or trade association) a company has joined (if any) and if that association has a wage agreement with a union. A logistics worker in the mining industrie (they have quite large storage/distribution departments) will be payed by IGBCE(1) since the german mining companies have a treaty with that one. One working for Volksthingies will be IGM(1) since car manufacturers made treaties with those. And the poor guys at Hermes are with Verdie so they are underpayed (verdis logistics tarrif is a piece of shit) and overworked.
Since we have "liberty of coalation" for both workers and employers a company can always elect NOT to join any trade association. In that case any wage agreements are done between the company and the employees. Amazon has gone that way as have most IT companies (BITCOM is not working as a trade associatian and no engineer wants verdi as a union(3))
(1)Industiral Union of Miners, Chemical Workers and Energy/Powercompany workers
(2)Industrial Union of Metalworkers
(3)One look at verdi's origins and key personal and you know they do not care for you, won't understand the needs/ways of anyone not working 9-17 (1h lunch brake, state employed preferable)