From what I understand, you're not supposed to turn the box inside out. If they catch it, they'll charge you (or your customer) the additional postage.
"Express Mail and Priority Mail packaging provided by the USPS must be used only for Express Mail or Priority Mail, as applicable. Regardless of how the packaging is reconfigured or how markings may be obliterated, any matter mailed in USPS-provided Express Mail or Priority Mail packaging is charged the appropriate Express Mail or Priority Mail price."
http://pe.usps.com/text/DMM300/601.htm#1064805