so you are not double charged, yet.

these are two separate rental contracts ( as you made two transactions thru priceline), you can NOT assume your  second term rent is the coninuous part from the first one.

from acoounting point of view, the charges now hertz is asking from you is the late/over-use charges form the first rental, that is the different type and rate to the normal rental cost from the second rental.

 priceline is only the middleman, they dont care about the nature of your rental contracts.

 you have to talk directly with hertz,  see if you can get them negotiated on teh charges.