That 'last' message wasn't part of your original post
not that it matters.
Your dealing with a company in a Country where English is a second or even third language. They explained it as clear as they could that it was your fault and why it was your fault. You wouldn't let it go. Your comments in your 'side note' started and ended it for you. So they sell a few items in the US, they sell Globally, should every person who deals with them expect them to have everything listed in their language no matter what Country they come from?
Then you took another go at him with your 'proof' reference. I don't think he needed you to point out he had the wrong word. Not what I'd call 'civil and polite'.
You should have checked your invoice, even if it was 'auf Deutsch' as you put it. If you didn't notice the 3 items missing then when you came to pay, the $19.45 price difference should have told you something was wrong somewhere, but it didn't. It was easier to blame them and put the error at their door.
'Antik-Corp Review (Terrible!) Do not buy from them!' was the header you used, I'm sure their sales won't drop because of this, and it shouldn't.
Life's to short, let it go and have a beer or two