As most of you know, Mac OS X comes standard with a utility -in the context menu- to compress files quickly and easily, but if what we want is something more advanced we have a very good option in Entropy.
Advanced
The great virtue of Entropy stands out in that it allows you to store files with really useful options such as high-level AES-256 encryption, something that to gain security we can go luxury. In addition, it allows you to filter unnecessary files such as the .DS_Store or view the contents of a compressed file without having to extract it.
Seeing what it allows and what it costs, It seems to me that the 15 euros are not justified at all. But it is up to the developer to choose the price and he will have a reason for doing so, although in my humble opinion if he lowered that figure a little it is possible that he would optimize his sales.