IMHO, that is not uncommon for Avis at a hub airport like IAH. I have run into exactly the same issue at CVG and ORD in the past (at ORD, it was "no two-day rentals, but three day is available but not for one-way). At CVG, I simply use someone else. At ORD, it was late, the agents were snotty (they made me call on the phone and make the reservation while they watched, then when it showed in the computer they rented me the car) - so I rented the car and dropped in STL. IIRC, I had to pay a pretty big drop fee, but the potential loss of business was many times the cost of the drop fee.