I have reservations about asking permission for mentioning any real product (though I do take Lin's point about the possible marketing benefits!). It is not a legal requirement to do so if you are simply referring to a product in passing (though obviously if you are going to denigrate it you need to be careful!). I don't suppose Stephen King asks for permission every time he mentions a Hershey bar in his books, for example

If you are quoting something like song lyrics, you do need permission, of course, unless it is out of copyright.
But my main point is that you shouldn't let yourself get bogged down by this, or worry that you can't write your book until you have permission from anyone whose products you may have mentioned. OK, you need to take reasonable care, but publishers have legal departments whose whole job is to worry about this sort of thing! The main thing for the author is to get the book written

Nick GM