Here is the code that I used for IFC to flip their Eltron Labels (which I believe is the only viable option for WWEX Engine for UPS Parcel):
"Globals.CurrentPackage.Label.PreLabelC2 += "ZB" + Environment.NewLine;"
*ZB will print from Bottom-To-Top
*ZT will print from Top-To-Bottom
Zebra ZP450 worked great for IFC and didn't need to flip afterwards (ZP450 uses both ZPL and EPL, as a note). As a note, for the UPS Parcel Doc Tab portion, using the 4x6 w/ the 1 inch Doc Tab required me to NOT use the Standard Product Doc Tab settings and to override with my own. You can scavenge from the IFC Integration (PreLabel IntPoint) or print to a file w/ the Doc Tab settings and alter-and-add to the Integration.