AVIS Rent A Car Do NOT give AVIS gift cards...save your money! Internet

I received a gift card to use for a car rental from AVIS.  This is an AVIS gift card for $50 that was meant to be used as payment towards a rental.

I made a reservation online for the Atlanta airport.  Once I got to the rental counter, they said they could not accept the card since the number wasn't added to the reservation at the time I made the reservation.  That made absolutely no sense to me, but I figured fine, I will use it another time.

So, the "another time" has arrived.  I go online to avis.com/giftcard to make my reservation as I was told.  I tried to put in a discount code I use all the time from my airline, but it would not allow me to use it.  I got a quote of over $500 for a premium car for a week if I use my gift card.  Now, if I just go to the website and make a regular reservation using my discount code, I would pay only $300 for the same vehicle for the same week, but they will not accept my gift card.

This seems illegal to me.  The card is was paid for by cash, so how can the rental car company charge me almost double to use it?  I am basically giving this company $50 and getting nothing in return because it is not to my benefit to use it with their system.  Any suggestions??