It's not upto MagicBox to tell you what you can and can't talk about.
If you have any contractual obligation at all, it will be for exclusive rights for sales...
I released my first pdf on magicbox for £8.99. Nobody knew my name, but I had a lot of quotes and good press from the Blackpool magic convention.
If I was you, send it over to someone.....who has been in your position.....
It is your first release, so forget about money. It's not going to make you rich... you need to concentrate on building a reputation.
The best way to do that with good material is to get as many people to see it as possible.
Baring in mind that Magicbox will take 50% of all profits.
Stay under a tenna for a first release mate. Seriously.