From the ShippingEasy knowledge base:
"For both Domestic and International, only the 4" x 6" Label or 4" x 6" Label (w/ Packing Slip) options, for printing to 4" x 6" label paper, are compatible."
This is true, but I don't understand why. I like to print my labels 2-per-page, so I have to go into the settings to change that any time I get a multi-piece order. Seems like that should be an easy fix.