Go with First Class. The only real advantage offered by Priority Mail is the handy-dandy box, and it ain't worth the price they charge for the service. And you don't need signature confirmation -- I order from several online retailers, and none of them use that.
Go with the cheapest shipping rate, and add $1 per order for shipping and handling. No one who has ever prepared a bunch of packages for shipment is going to begrudge you the extra buck.