Would it be possible to add support for the USPS Global Express Guaranteed (GXG) shipping option?
I don't see it mentioned on Express1's site, by Endicia supports it:
Sometimes we have orders that ship in boxes too large for USPS Priority & Express, but GXG allows the larger sizes. (ex: 24"x15"x24" to Australia)
UPS supports that size, but the rates are significantly higher then USPS GXG.