i typically try to use the USPS as i've had really good service with them and the cost is good where i live.
i also try to find a taylor box to ship as they are built 1 inch "under" the volume rule of 108 inches that most places upcharge as oversize. taylor boxes had this marked on one end-lately the measurements on the box are not there, seems like it was inked as 47x9x21.
now if she measured it wrong and that sounds like it, i would ask for another clerk or point it out to her-some folks throw the tape measure up haphazardly and get the wrong measure. if the box is distorted it may be out of shape giving a wrong measurement also. even if it came out to 48x9x21 and was oversize, they still ship but tack on an extra charge.
so options are:
1)take it to another USPS and be sure to stand your ground and even take your own ruler and show them(check at home and make sure the box is in the right size 47x9x21)
2)take it back and ask for a remeasure or maybe another clerk will be present and do it more correct.
hope it works out for you
d