Overcharged, gas tank near empty upon rental

Reserved on-line.  Overcharged upon pickup from on-line quote. Thrifty wanted to sub a compact for full size.  Gas tank had less than 1/4 tank at time of rental and of course you sign rental agreement before you get your rental car.  We filled  before returning to avoid the $7.00 per gallon charge.  The agent did not seem to understand our complaint.