Looks to me like Sprint sends out the bill at the beginning of the cycle and expects payment before the cycle is completed. What that really translates to is paying the bill as soon as you get it isn't necessary and takes interest from you. Paying just prior to or on the day it's due makes the most sense as it's still within the monthly cycle.
1500 EDF with 27% discount should end up being somewhere around $95 before taxes and fees so somewhere in there you must have gotten some other type of credit.
Just note that if Sprint made a mistake, you are still liable for the actual payment and they have a certain amount of time to correct that mistake and expect payment. Chances are though, someone who put in your billing just made a mistake and would not be going back to check on it.