Getting invoices despite cancelling the service in the written form in the NOS store 10 days prior

After numerous attempts at cancelling the service through the phone/website, I decided to go the the nos shop directly to do so in writing, as someone on the internet suggested. After this, I asked NOS staff which was assisting me: “Am I good, will i need not to pay for the following month?”. To which they replied: “Nope, only the cancellation fee of 75.58 euros”. I rephrased this question numerous times just to “be safe”. Lo behold, billing cycle comes, and I get another invoice as if no cancellation happened. Basically I need cancellation of the invoice now as well. Please help, I don’t want to go to nos store anymore, it is not nearby at all. Its been extremely frustrating experience with service cancellation already, and I just want it to be over. Just let me pay my cancellation fine and be free of this company. Thanks.