Technically this is stealing (Priority boxes are included in the cost of priority shipping, they aren't free), so I can't recommend it. And since USPS essentially is run like a non-profit, the only way to pay for this type of theft is by raising rates for everyone; it's not coming out of some fat cat d-bag's pocket.
If you do choose to do this, be sure not to use Media Mail because it is subject to inspection. Also, I wouldn't recommend doing a bulk shipment like this because if the postal employees catch on you will either have to repack every package or pay for priority shipping for all.