jazz-style pack and sandbox?

[dflat]

I just want to ask if there is a other way to buy the in-app (jazz style pack) ?
Why I can not pay with the same system that I have done for purchasing iRealBook?

Purchasing the style pack(s) uses your iTunes account which is what you used for purchasing iReal book. It is contained inside the app as it only works with the app.

I don 't know how to use the sandbox ....

The sandbox is to practice sending your files to the forum or if you need to use it temporarily to move a chart/playlist to one of your devices (although this is normally done via email to yourself privately.)
More help here:
http://irealbook.net/
under the Help menu.