magazines can be sent parcel post (any reading material) which is less expensive...depending on weight it is sometimes less expensive to use a different package than the flat rates...the flat rate packaging is great if your items weigh quite a bit, you save that way, if it is very light it is more cost efficient to use regular packaging since the flat rate is...flat rate...if your package only weighs 1 pound it would cost alot less to send it out of that flat rate box...but if it weighs 10 pounds you will save going that way.
the usps web site is user friendly and you can put in different size packages and weights and check all of the options available to see what is most cost effecient.