Debt collection trouble after using a scratch card

Hi, I have a big problem. About a year back, I received a kind of scratch-card with some freebies (approx. worth of 10-15 Eur). I normally throw such things, but it was just a small postcard and there was no mention about any cost etc and so I filled it and sent it back. After that I changed my residence and now yesterday, I have received a letter from Inkasso as 'LETZTE MAHNUNG' and saying I have an outstanding of approx. 300 Euros which I shoul pay by tomorrow.

Can someone suggest me how should I proceed regarding this.
1. I don't want to just ignore it since I plan to continue in this apartment for very long.
2. I don't want to pay 'coz I am sure there was no Contract mentioned in that postcard - I kept a copy of the same for a few months as a proof, but then I threw it.
3. Can I approach some customer-safety-offices in Frankfurt?
4. Should I call the agency and try to explain them in my grundstufe-level Deutsche? (But I fear it may lead to further problems).

Please help me. Thanks.

Definetly go to a Verbraucherschutz-Organisation in Frankfurt, sounds like you are a victim of a scam.
This is what I found online for you:

Verbraucherzentrale Hessen
Beratungszentrum Frankfurt/Rhein-Main

Große Friedberger Straße 13 -17
60313 Frankfurt/M.
Fax: 069 / 97 20 10- 70
E-mail: frankfurt@verbraucher.de
Leitende Beraterin/stellv. Geschäftsführerin:
Gabriele Beckers

Good luck and act fast!

Cheerio

I suppose you are screwed. The trick with scratch cards are one item will be free and you will have to pay for other items they send along. But in any case, you should have received a bill before receiving the letter from Inkasso.