I'm a buyer and I generally sort searches by lowest cost and postage.  However....

As the cost of postage for FREE POSTAGE items is factored into the cost of the item it really only works if you are buying a single item otherwise if you are buying more than one you are paying full postage for each item though they may be sent in a single parcel. 

So if you are buying multples then it's often better to look at postage as separate, especially if the seller combines postage or discounts extra items (eg ist item full postage, each additional item up to 5 is free postage or postage is a dollar per additional item etc).

I guess you need to consder what your customers buy - one off items or multiples, and how much difference the costs are.