I'm not familiar with German law, but I'd like to complain about a certain international company as I was tricked by a company to pay to use a method that was shut down by the company online and many people have been complaining about this in comments. I really don't want to go into the details of how things went, as that's what I'd like to bring to the specific authorities but are there any places (official authorities) for consumers to complain about an encounter in online shopping. I've also complained to the company.
You can complain anywhere you want. Facebook, Twitter, review sites, write your own blog, etc. It really depends on what you are trying to achieve.
The equivalent to the better business bureau are probably these guys: https://www.verbraucherzentrale.de/ I wouldn't get my hopes up that anyone would actually do something, if that's what you want.