Безопасен ли tdc.dk?

I had an abonnament at them but because they were asking for more money because we payed late (because the letter didn't even came to our mailbox in the first place) we decided to move to another company. We had payed for 3 months and the last month they should have payed us back the money, from around 500 kr how it should be for one month and the letter they payed us back around 150 kr. This is unnacceptable and we are very dissatisfied with their services.