Most are free, a few are limited trials with no easy in - app payment system, which gets weird. (cnet.com)
For example, you may not offer access to metadata or cover art conditional on the purchase of tokens or credits or any other in - app payment feature. (law.stackexchange.com)
All of the content that is available on the service has all advertising and in - app payments removed, to make sure the little ones don't rack up the bill. (goodereader.com)