Contact issue (English)

I hope someone can help me fix the situation I'm in, and help me in English! Thank you in advance. What happened is that I tried to extend my contract last week and also replace my broken phone. o2 offered me a Samsung galaxy s7, he said it was a special offer for me as a long time customer, I would continue to pay around 40 euros per month for the contract with phone. I was happy with that and accepted. Next day I received an email "confirming" that I had ordered a new sim card and phone - in other words a second contract. I only need one. This is the second time this has happened, it seems I cannot trust the o2 sales team. Last time this happened I was told it was a "rogue" agent who had since been sacked. Although I speak some German, as an English person living in Germany I find it much harder trying to arrange things like new phone contracts. So now I have the S7 and sim card at home, unopened. I need to send them back. How and where should I do this? Will i have to pay the postage? I have now extended my contract correctly and have a moto g3 instead. The sales person for the g3 said it was too late to stop the s7 being posted, but she would cancel the contract for the other sim. I am a bit concerned in case this hasn't happened. I am really annoyed with o2, I don't have time for all this in a working man with a buddy life, it usually takes me half an hour to reach anyone in the phone.