What is the version of eCart that you are working with? This looks like an old issue that has been updated in the newer releases of eCart. I'm including a new updated copy of the USPS file for you. Make a backup of yours and replace it with this newer file and test it out again.
The problem with the line of code is the dot dot where it is assigning the value to this variable.
This
$USPS_XML ..=
should be this
$USPS_XML .=