Upvote:9
You are wrong in your assumption that you can carry any goods between EU states without paperwork. For certain categories, taxes may still apply even if there are no customs duties, because taxes are not harmonized.
If I found that with a few moments of googling, you would be expected to do the same. If I had to guess, they saw the fact that you maxed out your vodka limit as a sign that you knew there are some limits.
As to complaints, of course you complain about German customs in German courts, unless you want Her Majesty's government to start diplomatic proceedings on your behalf. If you upload those forms, with personal identification blanked out, there may be a paragraph where exactly you can complain.