Hallo Alexander,
At 19 years old, this is a bad experience for you.
You must learn from it 
Unfortunately, you cannot put an old head on young shoulders
1) To retrieve the situation in your first question.
It is sometimes best to speak quietly over the telephone when there is a
misunderstanding. You need to know if the researcher actually looked for the
data you need. If he did search, but couldn't find it, you need to know
where he looked - then you know at the very least that you should not waste
further expense looking for it.
2) You received good advice. Pay the money (#2) quickly - it is a 'sellers'
market.
(sorry - I cannot recall the German for the verb "to sell")
3) 20 euros is not expensive, it is cheap.
Rena in England.