Did you select USPS as the shipping type? In the file I have with this name the line of code referenced is just checking on the package weight. Here is the line of code that I have on the line referenced:
if ($USPS_LbWeight > 0 || $USPS_OzWeight > 13.0) {//weight limit for first class
Is this the same line of code that you have on line 49? If so then ther is probably a bad value that is being used within this server behavior. Are you using First Class mail as the type of mailing for USPS? Please post back with this info and if we need to look into this further I can open a ticket so we can exchange the files in private.