Just set the maximum delivery cost to 0.00 in your account... then they will fail-over to delivering via WIFI by default.
similar to choosing to send them to
YOURNAMEHERE@free.kindle.com.
I vaguely recall there being some special case about periodicals but I forget the details... I always convert everything I want delivered to a "book" anyway. Which is almost never, but I digress.
HTH.
Periodicals merely refers to them being released over a (generally) fixed "period", hence the name. Harry is right. Well, he gets my vote 