Join Date: Jun 2010
Location: Melbourne, Australia
Programs: QFF NB, Velocity Silver, Hertz 5*, Avis Preferred, Thrifty Blue Chip (Oz), Europcar Privilege Exec.
Posts: 378
A contracted rate is a rate agreed between Hertz and in this case, Amex. The rate does not fluctuate based on demand, so even when demand is high and prices are high, you get the benefit of a negotiated rate, which usually will be lower. Contracted rates often have other benefits, like insurance.
All Hertz contracted rates are subject to change (your original reservation confirmation will state this), which occurs when the contract is renogtiated, but this isn't very often (usually only once a year). It sounds like the new rate applied from July 1.