What is the required amout of time that a utility company must give for consumers to pay their bill without a late fee.

Our water billing company (ISTA) is only allowing a 7 day grace period from the billing post mark date to when they are requiring us to have payment to them. We just recieved our water bill today on the 27th with a post mark on the 24th. The bill was generated on the 14th making it appear that we have 15 days or more to pay the bill. Is there a law that governs this type of billing abuse. We have even tried paying ahead to avoid late fees. They end of taking the credit and appling it to fabricated late charges.