This offer isn't exactly "free". If you read the ToS, you will see that you will be charged if you don't cancel the offer at the end of the trial period. However, if you can't download MP3s anymore, it means you have reached the limit of 50 free MP3s. In this case, I recommend on canceling your account and then use another methods to get music legally. You should uninstall this offer as soon as the trial period is over to avoid charges to your credit card/account.
