ok I think I got it.
This is the error I got:
UspsCom::DoAuth API=RateV4&XML=PARCEL7911010REGULARTrue 80040b1a Authorization failure. Perhaps username and/or password is incorrect. UspsCom::DoAuth
One thing that has always bothered me is that the checkout wizard never asked for a password, I do enter the username they sent me but...