Sometimes the flat rate is not the cheapest way to go. My experience is that it really has to be packed full ( I mean really full) to be of a cost savings. Every time I have used one I ask what the cost is if its not the flat rate. From experience I learned only heavy items are really worth it. Many like the convience plus the free boxes of the flat rate.
Remember , magazines, and books can go media mail ( much cheaper but slower) , so you may be better of sending two packages one media and the other with fabric regular first class or parcel post.